It all depeds on the Pokemon. If you has a Water Type and he has a Fire Type, you can beat him. If you have Pikachu and he has a Rock Type, then switch it because Rock Type can resist Lighting Type moves. (I have this and I can't beat Brock!)
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Since Gary uses a team of varied types, it's a good idea to have a team which has varied types. Your team's movesets should be varied, having attacks of different types. There's no specific Pokemon that are best to use, so use your best-trained Pokemon, whether they're regular or legendary Pokemon, and be sure to have plenty of Potions and Revives on you.
Gary
I think it is Blue. And the other way around if you are playing Blue yourself. In Yellow I think its just Gary.
Lorelei- ice Pokemon Bruno- rock Pokemon Agatha- physic/ghost Pokemon Lance- Dragon Pokemon Last your rival Gary
On the Pokemon television show, a character is named Gary Oak. He is the rival of Ash Ketchum.
Blue is the default name for your rival in the original game series of Pokemon (Red, Blue, Green, and Yellow versions). Your character's default name is the game is Red. In the animae show they then changed Blue's name to Gary and Red's name to Ash.