Are you looking for a way of describing somewhere that habitually gets crowded (the use of the simple present in your post suggests a habitual action) or a one-off incident of crowding? Please give us a full sentence in which you would like to put the phrasal verb you're looking for.
If the gym is in the process of getting crowded (i.e. there are quite a lot of people already and it looks like there will be more coming in, you could use: