Caribbean Coast and Coastal Skyline are both in Tung Chung and both have great gyms.
Very clean with all the gear, everything you would need for a good workout.
Just bang on the door and say your a visitor will cost approx $20hk.
If you live in either of the above then $10hk for as long as you want.
If you have a friend that lives at either they can get you in or borrow there card and say you live there as we all look alike.