Most gyms don't have experienced or trained people looking after the kids. The loop hole is that they are a child minding service so they don't need qualifications.
I also had a bad experience with my gyms child minding, I never took my kids back there. I have however had 2 of my 3 kids in registered child care and it has been a great experience.
Glad your mum is happy to have her for a bit longer so you can still go to the gym - plus it's a wonderful bond for your DD to have with her nana <3