All the comments about council approval are correct, including provision of parking spaces and change of use from a garage to a gym.
What is critical here, though, is whether or not your neighbours are going to object – they are the ones who will dob you in to the council if you don’t have official approval and they don’t like what you’re doing.
There are a few cases in the Flat Chat annals of people turning their garages into games rooms, rock band rehearsal spaces and TV lounges but it’s only when they annoy their neighbours, either because of excessive noise or because their car is now parked on common property, that the Council sends someone round.
A quiet word with your neighbours would certainly be worthwhile in the first instance, but if the Coucil sends someone round and finds a wall where a garage entrance used to be, they may have issues.
By the way, you can’t expect your strata committee to officially approve something that hasn’t or wouldn’t be OK’d by council planners.