Which region did Pokémon Gold and Silver introduce?

Pokémon Gold and Silver introduced the Johto region.

Gold and Silver were the main-series games of Pokémon’s second generation, first released in Japan in 1999 for the Game Boy Color. Johto added 100 new Pokémon, increasing the overall total from 151 to 251.

Johto is located next to Kanto, the setting of the first-generation games. After completing much of the Johto adventure, players can travel to Kanto, making Gold and Silver unusual among the main games because they include two connected regions.

Kanto is therefore the common wrong answer: it is the original region, not the region introduced by Gold and Silver. Hoenn arrived with Ruby and Sapphire, while Sinnoh arrived with Diamond and Pearl. Johto also introduced features such as breeding, a day-and-night cycle, and the Dark and Steel types.