Your gym bag is basically your mobile locker.. So just as one would keep and have what they might/will need for pre-game/event & post game/event in their locker U should have in your bag.
Remember this.. It's Always better to have it on hand & Not need it.. Than to need it & Not have it...
Water or Gatorade, extra clothes just in case, extra leotard, Grips and/or tiger Paws if you wear them, contact lenses stuff if you wear them, deodorant, a snack if your gym allows you to have a snack break, tape, pre wrap, nail clippers or scissors for that occasional rip, and hair ties, scrunchies, clips, and bobby pins.
Water, snack, tape, pre rap, grips, deodorant and anything else you want
usually whatever you need.
Like: Water bottle, powder, hair ties,
Whatever else...