Overview

Pokémon Gold and Silver were released on October 14, 2000 in North America (November 21, 1999 in Japan and April 6, 2001 in Europe) for the Game Boy Color (although it also supported the Game Boy Pocket and Game Boy). Development was done by Game Freak, publishing by Nintendo, and marketing and licensing from The Pokémon Company.
Taking place in the Johto region, the second generation of Pokémon introduced 100 new Pokémon. Besides several unique Pokémon and other minor details, Gold and Silver are identical to each other. However, to collect every single one of the Pokémon, players must trade between not only Gold and Silver, but also the first two games, Pokémon Red and Blue.
Reception
Well-received by critics with an 89% average rating according to Game Rankings, Pokémon Gold and Silver was a critical success. Gold and Silver are regarded as some of the best iterations in the Pokémon franchise, getting a ten out of ten from IGN, and high scores from many other publications. Together, Gold and Silver sold 6.5 million in Japan alone, while sales in North America beat previous sales records, selling 1.4 million copies in the first week of sales.

Pokémon Crystal was later released as an updated version of Gold and Silver. It came out on July 29, 2001 in North America, and was the only Pokémon game to be released exclusively for the Game Boy Color. Despite being largely similar to Gold and Silver, Crystal added many new features, including new series' staples such as the ability to choose the player's gender.
A remake of Gold and Silver was developed for the Nintendo DS. Pokémon HeartGold and SoulSilver were released on March 14, 2010 in North America, and on September 12, 2009 in Japan. It is predominately a graphical remake of the game, but it also brings all of Johto's Pokémon into the latest generation of Pokémon (despite all of them being available across many of the later games released). The remakes also have new modern features added from the fourth generation of Pokémon games, new characters, and special functionality with a device called the Pokéwalker.
Gameplay

Set in the Johto Region, Gold/Silver is a traditional role-playing game with gameplay essentially the same as Pokémon Red/Blue and Yellow. There are two different "types" of gameplay. In battles, players have several options. The Pokémon that is in battle can attack the rival Pokémon (be it a wild Pokémon or a trainer Pokémon) with a variety of attacks. The player can also use many items, including healing items, battle items, or Poké Balls (used to catch wild Pokémon, who upon being caught can be used in battle). Players can switch out Pokémon and run away as well.
The main goal of these battles is to get the rival Pokémon to "faint." When all of an opponent's Pokémon have fainted, the battle is won. However, when fighting a wild Pokémon, players can attempt to catch it for further use. Other trainers' Pokémon cannot be caught. After winning a battle, the Pokémon gain experience, and eventually level up. For many Pokémon, when they reach a certain level they evolve into another form of the Pokémon (e.g. Pidgey evolves into Pidgeotto, Totodile evolves into Croconaw). Some Pokémon have special requirements to evolve, such as having specific items equipped, or leveling up during a specific time of day.

Each Pokémon has a type, mostly based on nature (e.g. water, grass, fire, earth). Each type has a (or some) strength(s) and weakness(es). These types also affect the Pokémon's statistics, such as, defense, attack power, and speed. Some types are completely immune to another type's attacks. In addition to stat bonuses and immunity, many Pokémon have unique attacks available relating to their type. Gold/Silver were also the first games to introduce dark and steel type Pokémon, thus completing the 17 types of Pokémon that remains the standard in all games in the series since.
Overworld gameplay follows Gold, the protagonist of Gold/Silver. He can travel to the various towns of Johto region, along with many other locations. Players can interact with objects and non-playable characters, sometimes relating to the plot. In towns, players have a variety of options, including purchasing items, healing all of the Pokémon in the party (as well as switching out Pokémon), and challenging that city's gym leader. There are many other areas to visit, including caves, forests, and even the ocean. While in the overworld, it is also possible to trigger a random battle with a wild Pokémon.
New features of Gold and Silver are numerous. It implements a day and night system that is based on a real-time internal clock. Influenced by this clock are many things, including specific events and what Pokémon appear. With the ability for Pokémon to hold items introduced, many items were created to take advantage of this mechanic, including berries (which can do many things, including boosts to power, healing status effects, and restoring health). Pokégear, a new key item introduced featured a map, watch, radio, and phone, allowing the player to call non-playable characters (and to be called by non-playable characters) and listen to the radio at certain times (according to the real-time clock).
Gold and Silver's legendary Pokémon include Raikou, Entei, and Suicune, who are a completely new type of legendary Pokémon. Instead of encountering them at a specific location, they appear randomly around Johto, and will run away occasionally. However, they retain any status effects or damage upon running away.

Unique "shiny" Pokémon were added, which were a different color than the regular version of the Pokémon, and extremely rare. The Pokémon types Steel and Dark were added in Gold and Silver. Steel type are strong and have a high defense, while Dark type are powerful against Psychic Pokémon (they are even immune to any Psychic attacks).
Breeding was a feature that has stayed since Gold and Silver, allowing players to leave two Pokémon in the day care center for a chance for them to breed. Many species of Pokémon cannot breed, however. The child of the Pokémon will gain whatever moves its father has, while it gets the species of its mother.
Plot

Taking place in the Johto Region, Gold and Silver follows Gold, a young boy for New Bark Town. In the beginning of the game, he moves to New Bark Town and meets Professor Elm, who gives him the choice of one of three starter Pokémon: Cyndaquil, Totodile, or Chikorita. Later, Gold will run into his rival, Silver, and he will challenge Gold to the first of many Pokémon battles with him. After the battle, he will flee from Gold who will discover that he has just fought against the thief who stole one of Professor Elm's Pokémon. Gold will also meet Professor Oak, who gives him the Pokédex and the important task of catching all of the Pokémon in the Johto Region.
To do so, Gold must battle the eight gym leaders of Johto, each specializing in a particular type of Pokémon. After defeating each of the gym leaders and collecting their badges, Gold must fight the Elite Four and the Champion, and eventually the eight gym leaders of Kanto. In the very end, Gold fights the protagonist of Red and Blue, Red, in the final battle. Along this path, Gold must also battle the remaining members of the organization known as Team Rocket, as they try to re-unite and contact their old boss, Giovanni, to convince him to lead them again.
Pokédex
The Pokémon Gold/Silver Pokédex starts out as if it were separate from that of Pokémon Red/Blue, however after defeating the elite four, the player gains access to a national Pokédex that allows them to see all of the 'old' Pokémon from the original game. The first 151 Pokémon are as per the Pokémon Red/Blue Pokédex. The remaining Pokémon, those that are new to this game, are listed below.
152 Chikorita

153 Bayleef
154 Meganium
155 Cyndaquil
156 Quilava
157 Typhlosion
158 Totodile
159 Croconaw
160 Feraligatr
161 Sentret
162 Furret
163 Hoothoot
164 Noctowl
165 Ledyba
166 Ledian
167 Spinarak
168 Ariados

169 Crobat
170 Chinchou
171 Lanturn
172 Pichu
173 Cleffa
174 Igglybuff
175 Togepi
176 Togetic
177 Natu
178 Xatu
179 Mareep
180 Flaaffy
181 Ampharos
182 Bellossom

183 Marill
184 Azumarill
185 Sudowoodo
186 Politoed
187 Hoppip
188 Skiploom
189 Jumpluff
190 Aipom
191 Sunkern
192 Sunflora
193 Yanma
194 Wooper
195 Quagsire
196 Espeon

197 Umbreon
198 Murkrow
199 Slowking
200 Misdreavus
201 Unown
202 Wobbuffet
203 Girafarig
204 Pineco
205 Forretress
206 Dunsparce
207 Gligar
208 Steelix
209 Snubbull

210 Granbull
211 Qwilfish
212 Scizor
213 Shuckle
214 Heracross
215 Sneasel
216 Teddiursa
217 Ursaring
218 Slugma
219 Magcargo
220 Swinub
221 Piloswine
222 Corsola

223 Remoraid
224 Octillery
225 Delibird
226 Mantine
227 Skarmory
228 Houndour
229 Houndoom
230 Kingdra
231 Phanpy
232 Donphan
233 Porygon2
234 Stantler
235 Smeargle
236 Tyrogue

237 Hitmontop
238 Smoochum
239 Elekid
240 Magby
241 Miltank
242 Blissey
243 Raikou
244 Entei
245 Suicune
246 Larvitar
247 Pupitar
248 Tyranitar
249 Lugia
250 Ho-oh
251 Celebi
Pokémon Gyms
The system where the player has to get badges throughout the world to be allowed to enter the Elite Four and try to become the greatest Pokémon Trainer ever returns from Red/Blue/Green/ Yellow. This time the region where the game primarily takes places is called Johto and the gym leaders are all different than its prequel. Eventually, when all eight gym leader are defeated in the Johto region and the Elite Four is also defeated, there is an opportunity to go back to Kanto (the region of the first generation) to a replay on all past Gym leaders, assuring a total of 16 Gym Leaders and 16 possible badges.
Violet City Gym
Leader - Falkner
Badge - Zephyr Badge
Pokémon Type - Flying
Pokémon Used - Pidgey, Pidgeotto
Azalea City Gym
Leader - Bugsy
Badge - Hive Badge
Pokémon Type - Bug
Pokémon Used - Metapod, Kakuna, Scyter
Goldenrod City Gym
Leader - Whitney
Badge - Plain Badge
Pokémon Type - Normal
Pokémon Used - Clefairy, Miltank
Ecruteak City Gym
Leader - Morty
Badge - Fog Badge
Pokémon Type - Ghost
Pokémon Used - Gastly, Hunter, Hunter, Gengar
Cianwood City Gym
Leader - Chuck
Badge - Storm Badge
Pokémon Type - Fighting
Pokémon Used - Primeape, Poliwrath
Olivine City Gym
Leader - Jasmine
Badge - Mineral Badge
Pokémon Type - Steel
Pokémon Used - Magnemite, Magnemite, Steelix
Magohany City Gym
Leader - Pryce
Badge - Glacier Badge
Pokémon Type - Ice
Pokémon Used - Seel, Dewgong, Piloswine
Blackthorn City Gym
Leader - Clair
Badge - Rising Badge
Pokémon Type - Dragon
Pokémon Used - Dragonair, Dragonair, Dragonair, Kingdra
Vermilion City Gym
Leader - Lt. Surge
Badge - Thunderbadge
Pokémon Type - Electric
Pokémon Used - Raichu, Electrode, Electrode , Magneton, Electabuzz
Saffron City Gym
Leader - Sabrina
Badge - Marshbadge
Pokémon Type - Psychic
Pokémon Used - Espeon, Mr. Mime, Alakazan
Cerulean City Gym
Leader - Misty
Badge - Cascade Badge
Pokémon Type - Water
Pokémon Used - Golduck, Quagsire, Lapras, Starmie
Celadon City Gym
Leader - Erika
Badge - Rainbow Badge
Pokémon Type - Grass
Pokémon Used - Tangela, Victreebel, Jumpluff, Bellossom
Fuschia City Gym
Leader - Janine
Badge - Sould Badge
Pokémon Type - Bug/Poison
Pokémon Used - Crobat, Ariados, Weezing, Weezing, Venomoth
Pewter City Gym
Leader - Brock
Badge - Boulder Badge
Pokémon Type - Rock/Ground
Pokémon Used - Graveler, Rhyhorn, Omastar, Kabutops, Onyx
Seafoam Islands City Gym
Leader - Blaine
Badge - Volcano Badge
Pokémon Type - Fire
Pokémon Used - Magcargo, Magmar, Rapidash
Viridian City Gym
Leader - Gary
Badge - Earth Badge
Pokémon Type - Gary has a very varied team
Pokémon Used - Pidgeot, Alakazan, Rhydon, Exeggutor, Gyarados, Arcanine
The Elite Four
The Elite Four works pretty much like in the previous games. The player first faces the four members of the elite four, then battles against the Elite Champion for the title of greatest of all. Bruno from the previous Elite makes a come back, while the previously gym leader Koga appears as one of the four.
Elite Four Will
Pokémon Type - Psychic
Pokémon Used - Xatu, Exeggutor, Slowbro, Jynx, Xatu
Elite Four Koga
Pokémon Type - Bug/Poison
Pokémon Used - Ariados, Forretress, Muk, Venomoth, Crobat,
Elite Four Bruno
Pokémon Type - Fighting
Pokémon Used - Hitmontop, Hitmonchan, Hitmonlee, Onyx, Machamp
Elite Four Karen
Pokémon Type - Dark
Pokémon Used - Umbreon, Vileplume, Gengar, Murkrow, Houndoom
Elite Four Champion Lance
Pokémon Type - Dragon
Pokémon Used - Gyarados, Charizard, Aerodactyl, Dragonite, Dragonite, Dragonite
Legendary Pokémon
Legendary Pokémon are Pokémon generally found in a very limited amount in the world of Pokémon, so unique there is only one of each of them. These Pokémon have more experience than the rest, and can be found in already high levels ranging from 30 to 70. Like Moltres, Articuno, and Zapdos were the Legendary Birds of the previous games, this time there are Legendary Dogs, called Entei, Suicune and Raikou. There's also two new birds, Lugia and Ho-oh; although when purchasing one or the other version of the game one of these two Pokémon come printed on the cover, Both can be captured in each of the distinct versions. There's also a time-traveler Pokémon called Celebi that wasn't included in this version, and got removed from American and European cartridges of Crystal, was only obtainable through Nintendo Events.
Entei
Type - Fire
Level - 40
Location - Awaken Entei in one of Ecruteak's buildings and then chase him down across Johto.
Suicune
Type - Water
Level - 40
Location - Awaken Suicune in one of Ecruteak's buildings and then chase him down across Johto.
Raikou
Type - Electric
Level - 40
Location - Awaken Raikou in one of Ecruteak's buildings and then chase him down across Johto.
Lugia
Type - Flying / Psychic
Level - Gold 70 / Silver 40
Location - Lugia is found in the caverns south of Olivine. In the Gold version, he isan't available until the player has the Silver Wing in Pewter. In the Silver version, the player can just surf there and explore the dungeon until the player finds him.
Ho-oh
Type - Flying / Fire
Level - Gold 40 / Silver 70
Location - On the top of the Tin Tower after receiving the Rainbow Wing. In the Gold version the item is in the Radio Tower in Goldenrod. In the Silver version is it found in Pewter.
Celebi
Type - Grass / Psychic
Level - 30
Location - Illex Forest, only obtainable in the Japanese version of Pokémon Crystal, Nintendo Events or GameShark.