A concern here with the gym mats ...they are prob rubber or vinyl covered and either way like many materials will collect moisture, underneath between the mat and the wood and that's not good (for).. I know this first hand ...that's the very reason why I suggest a vapor barrier under the platform and if possible insulation .
Insulation could be done easily by lifting the platform . Glue foam board 1in thick under and use lath strips 90degrees to the foam screwed to platform every 2ft .. tapering the foam joints would help tremendously. And help midigate any moisture from the ground . Moisture from the ground , or air temp difference , what we all hate ..DEW.
A good example of what I am talking about is lift a rubber Matt that has been outside after a rainstorm , but not exposed to the rain .in a porch or garage.....it will be damp ...to stop this moisture, either a barrier with insulation , or if your climate is somewhat dry, insulation alone ..the temp difference slowed by insulating even small, would make a great difference in your maintenance, equipment longevity , and building. But that's my opinion.