I'm going to make a girl at the gym think I was creeping on her
Why would you want to freak her out like that?
I want to make her shape up. No pun intended. If she thinks there's someone creeping up on her, it would be more motivation to get in shape to deal with creeps.
Shaping up is always the right thing to do at a gym. Creeping on her is just not right, you could be reported and then banned from the gym. Stop that and be civil towards everyone, we deserve to have our own space.
She could be so hot if she just improved her body. I'm going to tell her that I would fuck her if she got more in shape.
I think that you should just keep to yourself. It is wrong to make someone feel that uncomfortable intentionally.