Which two types did Pokémon Gold and Silver add to the Pokémon series?

Pokémon Gold and Silver added the Dark and Steel types to the Pokémon series.

Released in Japan on November 21, 1999, Gold and Silver were the second-generation main-series games. They expanded the battle system beyond the 15 types available in the original games by introducing Dark and Steel, creating new strengths, weaknesses and strategic combinations.

Dark-type Pokémon and moves were designed in part to balance the powerful Psychic type. Steel-type Pokémon, meanwhile, became known for high defensive strength and numerous resistances. The games also added new moves that used these types, giving battles more tactical variety.

These games introduced much more than two types. Gold and Silver also added a real-time clock, held items, Pokémon breeding, the Johto region and 100 new species. The Fairy type came much later, in Pokémon X and Y, while Dragon was already present in the first generation.