We unfortunately do not have a/c and it's HOT and humid most of the year. Our gym is actually a warehouse with garage doors and sometimes they open thoses but I don't think it makes much of a difference.
DD has never complained of the heat, though. Probably because she doesn't know any different.
DDs gym has A/C and tons of huge fans. It was 110 degrees outside today and for the past week, so that is a good thing. The viewing area doesn't have air though, so we freeze in the winter and are dying in the summer. At least our kids aren't too bad. The air isn't super cold, but I can see why, it would cost a fortune. Her old gym didn't have air and those girls were sweatin' like crazy.
No AC for us either. We only get short bursts of hot weather(above 90), so in the long run probably not worth it. They use swamp coolers in all 4 corners of the gym and keep doors open as much as possible. Yes, the girls are strongly encouraged to drink lots of fluids.