Sometimes guys like these hang around my club, too. For me it's a matter of timing.
If it's the first time I've seen the guy I might quickly look, (because it's pretty clear he's showing off) and I'll simply "notice" him in the room, look him in the eye, and nod and/ or smile. If he's still hanging around later, I might chat up some surface conversation- about the weather, something he's wearing, a workout technique he was using on the gym floor, etc etc...
If I see him again (later on or a return visit) and it's just him and I, I will look him in the eye, smile, and chat again- now that the rapport is more casual between us, and then just feel free to hang around him as he hangs out nude. If it is just he and I, I will be more blatant about looking him in his eyes, then back to his tool, and then back in his face- with no shame.
Sometimes I will use the mirrors in the club- because if he's preening in front of the club you can make eye contact using the mirror instead of directly. He likes his body, he's proud of it- and he's showing it off to those who are confident enough to look at it and admire it. He's rockin out with his cock out--- so enjoy it for what it is. It's clear that he doesn't mind a little cock appreciation- so if you want, and you want to risk being seen by other guys who are around you also dressing... then go ahead and look.
If it's done with a mutual respect and not in a provocative manner (after all you're in public) then look away, then look back, and enjoy the masculine energy that flows between you. Beyond looking- take it outside and elsewhere- unless you are in PRIVATE other 'male bonding' can get you arrested.
Again, it's about timing. Since this is a social place, if you are a regular and EARNING the right to be a 'regular' by having a regular schedule and putting in honest workouts, you will make friends who are on your same schedule, and probably your social interactions will be welcome by others. This will give you reason to carry on a conversation with other guys- even those who are bare- assed at the mirrors, applying lotion to their naked torsos before they pull on their boxers...