My recommendation is that you NOT do this if you're a novice at this type of thing. The most important reason is because its a surefire way to get your face punched and your ass kicked if you try to do it with a straight dude who's not at all into gay/bi stuff. I've seen this happen. It would be very different if you knew for a fact that the place had a reputation of being a meat market type of gym and/or that gay or bi guys regularly train there, but otherwise, don't guess. Some super-aggressive 'alpha' types will take serious offense if they find out that you're checking out their ass or coming on to them. The one and only, sole exception to this is if you notice that the dude is checking YOU out each time you look his way, or if he approaches YOU first. And unless you're a hot muscle dude yourself the chances of that happening aren't all that great.
I've been a part of gym culture for a long time, and what I know is that (for the most part) hooking up with men there is not as simple or as easy as those fantasy gym scenes in gay porn---although its probably different in gyms where mostly gay guys hang out. Most of those guys with the really awesome looking bodies go to the gym to get their workout on, not to hookup with anybody. And the type of buddies they tend to hang with are other guys who look like them and who take the bodybuilding lifestyle just as seriously as they do. Many will also not appreciate you walking over to disrupt their routine by trying to make friends with small talk.
I've only done this a few times over the years and it happened only after I had known the other dude for a while. I will tell you how you COULD go about doing this based on my experiences. You have to be into the lifestyle and have a great looking body yourself. Then, just as you're noticing them they'll also be noticing you. Many would never admit it but builders are always checking out other builders. The straight ones aren't looking at cocks and asses, but they're looking at mass, form, and definition and comparing. Even the ones who are bi-curious won't make it all that obvious that they're bi. Sometimes you may think they're watching you when its actually your routines that they're watching. That's why the 'eye contact' thing isn't all that reliable either. But its easier to meet and talk to them when you're 'in their league'---so to speak. In this case you can walk up to them, offer to spot, or ask for a workout tip, or tips about which gear to use to bulk, lean-out, or cut with. Whatever BS... Builders love to talk about this shit with other builders. The key is to act as though you are really interested in the workout, not in making friends. The buddy thing tends to develop naturally on its own after that. How you get to the point of them letting you fuck them is another game. In all of the few times that I've hooked up with a dude I met at the gym it was at my house during or after some type of game (usually football, UFC, etc.), pizza, and LOTs of beer and liquor.
Not worth the hassle if you're really not into the bodybuilding lifestyle yourself. If all you truly want is a hot muscle boy to play with, without the bullshit games or wasting time, its easier to try your luck with a hookup website (as long as its not Craiglist). I've tried the Craiglist thing a few times and the guys on there are pretty gross. LOL. Met some amazingly beautiful married dudes and bi M/F couples on Ashley Madison though.
Caveat emptor:
1. If the dude is somebody that you actually like and want to keep being buddies with, don't go there with any of this stuff. After the fucking is over things tend to get awkward, and the friendship is pretty much over. This happened to me in every case when I've done it, which is a shame because some of those guys could have been good friends.
2. If you're going to get into the business of fucking straight men (especially those who have no experience at being a bottom) you should be prepared to deal with a particular hygiene issue that may come up while you're back there.
