I'm going to tell my girlfriend to stop working out.
Why would you stop her from working out? How do you think that would be fair?
I don't like her working out around other guys and we had lots of fights about this.
Even if there are other guys at your gf's gym, it isn't okay for you to be jealous and make her stop exercising there. Work on your own jealousy instead of trying to control her.
I can't do that since I'm in a bad mood anyway.
You have to learn to control your jealousy because you cannot tell her what to do. If you do this you will be seen as overbearing and that is not attractive.