Is there an issue you are trying to “work out” right now in your waking life? Whatever is happening in the dream will likely reflect how well or how poorly you are currently dealing with this issue.
A gym is also a place of competition so ask yourself what area of your life feels competitive. That’s the area of your life this dream is addressing. The condition of the gym as well as what happens in the gym will directly relate to your current circumstances in waking life.