I suppose it depends on your gym- ours it totally oversubscribed and has a huge wait list for rec classes so you can't do more than one.
Personally, I would maybe think about something complementary to her gymnastics, ballet, dance, yoga etc. There's plenty of time for multiple classes per week if and when they make team! In the long run, ballet or dance might well be more help to her gymnastics in future than another hour of rec gym.