Just one thing to check - if you do get a gym ball, get one at the right height for you. When you are sitting on it, your hips have to be at least 5 - 10cm higher than your knees to bring baby into the right position. eg a 55am gym ball would be too small for me to do any good, I have a 65cm one, and that's just OK for me as long as it's fully pumped up (and you actually need to measure the circumference -I'm 5 ft 7 and 65cm just raises me by about 8 cm) if I was any taller I'd have to order a 75cm birth one, as I've not seen any gym ones that big.
Gym ones are made exactly the same way, but if it's too small, it just won't be doing what it's meant to in terms of foetal positioning.
HTH xx