My girlfriend has a gym membership that I'm going to tell her to cancel.
Why would you want her to cancel her membership at the gym?
I've seen the way she looks at the other guys in the gym, she's flirting them.
If you are jealous of her looking at other men, you are feeling insecure about the relationship. You should just trust your girlfriend and have faith in her loyalty, otherwise the relationship will be shaky and will eventually fail.
I trust her but I just want to be sure. I can't imagine her being with another man, it's driving me insane with jealousy.
Your jealousy is your issue and that does not allow you to micromanage and have control of your girlfriend's life. You should consider speaking to a professional about your jealously issues and in the meantime trust that your girlfriend isn't doing anything wrong.