Wait until she’s not busy (resting between sets for example) and then just be direct about it. You’re both there to work, so don’t try to draw it out and take up her time with a conversation (if one starts naturally that’s fine, but don’t make her feel trapped when she’s there to work). Literally just say “Excuse me, I saw you over there and I thought you were really pretty and wanted to say hi. Introduce yourself, then ask for the number/snap/whatever.
Just start talking, get on a first name basis, and after a few sessions make a move once you sorta know each other. If you jump straight to asking her out, she’ll feel pressure since she doesn’t know u and u go to the same gym. Trust me, just chat with her a little a few times before so she knows ur cool first