DH is really fussy about having a gym. The brochure says it's small with a treadmill and a bike. I'm trying to get some more information from someone who's been there and seen it or even better, used it.
No I haven't, sorry. But Neilsons are a smallish family firm based down here so I would contact them and ask. I know someone who works for them and they go on a lot of holidays 'checking out' the resorts and facilities, so I would think they'd be able to put you in touch with someone who would know.
I can't remember if this was the resort we visited but if it is and you are going because of 'advanced' dinghy sailing, it didn't live up to brochure because of direction of prevailing wind coming over land wind in the bay where they permitted you to sail was shifty and inconsistent. we did manage to persuade them to take us out of the bay which was lovely but for the parts of the week we were hemmed into the bay it wasn't possible to play on trapeeze boats etc as just kept falling over and in!
be aware that Halkidiki is adults only except high season. It's not generally a windy place so is good for beginners. I doubt that the gym would have much (although it should be as advertised) as it is a shame to go all the way to Greece and be indoors in the gym!
go this year if you want to go with Neilson, they are pulling out of this place at the end of the season.