If I had a hankering to use a gym again while still pre-op, I would check for a private changing room before trying it. Or I would do what I used to do last time I had a gym membership: change at home and drive in my gym clothes. I was still closeted at the time, but it was just more convenient.
Post-op, there would be no issue.
Swimming pool, though, I'm not even going to go near while pre-op. Nope, nope, nopity-nope! I'll think about swimsuits after my GRS.