Hmm...In principle I like the idea. I sometimes have long layovers and always have just carry-on luggage that includes my gym stuff.

In practice, though, I'm often too jet-lagged or have work to do or might prefer to do other things when passing through. So I'm not sure how often I'd use it.

There's also the question of whether other good things could be done with the space that would be set aside for this. A spa like TG in Bangkok or an FCT like LH in Frankfurt would be wonderful, though both require more space than the gym likely would. And of course the spa is only really worthwhile for TG F passengers and the FCT is restricted to LH F passengers.