Since this is the only Gible available in this generation of Pokémon games, you’ll need to trade it from Black 2 to White, Black, or White 2 if you want to use it in those games.
Once you have the right Friend Code, you’ll have a 50% chance of encountering a Level 30 Gabite.
Make sure to sneak by gently pushing the circle stick to move when you get close to where the Pokémon is hiding.