Well a gym isn't agricultural but if you don't change its ability to be used for agriculture, who would care? (Planners do not go out looking for problems.)
However, things like insulating or - my pet hate - occluding ventilation eg., by cladding open sides, would impact its ability to be used for agricultural use.
But the same thing applies, planners do not go out looking for contraventions, it would need someone to report it for the planners to look into it. (True wherever I've lived in England, and I doubt anywhere else can afford for planning officers to go out looking for contraventions either.)