I can't find it anywhere is it in Nimbassa city?
Yes, it's in Nimbasa City. If you're talking about Black 1, she's in the amusement park. It doesn't look like a normal Gym. I had trouble finding it too. To confirm which one it is, her gym is a giant roller coaster ride.
How to change your username
How to add a user avatar
Why was my post hidden?
The points system
Flags vs Downvotes
Type a Pokemon name, type, move or ability to see instant results.