How do you beat the first gym leader (in White) with a grass type Pokemon?

Before you challenge the first gym, go to the Dreamyard off to the right of the city. Talk to a girl and she'll give you a small monkey Pokemon that's at a type advantage to the gym leader. Just level it up once or twice through battling wild Pokemon, and it should be easily able to beat that pesky first gym leader. All the best luck~

I had a problem just like you, before you vs the gym you go the Top right corner of the town and go into the forest which called the Dreamyard. And you have to battle the 2 trainers and at the end of the road you will a guy standing there you talk to him and he give you, a monkey Pokemon that's water type

The Pokemon you get in the first Gym changes; if you picked Snivy you will have to battle Pansear (The Fire-type monkey), if you picked Tepig you will have to battle Panpour (the Water-type monkey) and finally if you picked Oshawott you will have to battle Pansage (the Grass-type monkey). They do this to try and make it harder for you but they do give you a bit of help; in the top right of Striation City you'll find the Dreamyard where you can get one of the monkey's above to help you; for instance if you picked Snivy you'll get Panpour to give you an advantage against the first Gym's Pansear (same for Tepig - you get Pansage, and Oshawott - Pansear) and you can keep it for the rest of the game if you wish. Just raise it up a couple of levels in the grass and Bob's your uncle. Hope you understand and it helps!

Those "monkey" Pokemon are a waste of time. Just thought I should warn others, "monkey" Pokemon suck in later parts of game. Was hoping to get other options but guess not.

Just use the monkey for the first gym then release him

The monkey Pokemon is realy handy, but I did the gym with just my oshawott. Just level him up. You can do both ways, monkey and starter Pokemon.