If I had a 6 hour layover, of course access to a gym would be great but in reality I don't really want a 6 hour layover and would much rather have a 1.5 to 2 hour transit and get going to where I want to go.In most cases CX is good at that I think. I've also been surveyed quite a lot in the last week and my take is that the more you fly (and you are the bigger revenue spenders whether in F,J,Y) you just want to get to where you want to go with the best efficiency and yes some nice lounges along the way to make you feel that CX value your business.
An F class type lounge like Frankfurt may work actually but I don't think a gym will get many people apart from the novelty factor.