"Hey! How's it goin'?" fits a lot of situations in the US, once we make eye contact.
But if I'm in a bar with a TV playing, I might make some comment out-loud about a news item, or sports moment, see if the guys near me will make their own remark. Or start a conversation with the bartender about almost any subject, hoping a customer will join in. I've found most guys in a gay bar by themselves are looking for a chance to talk, so I provide it (or I did when single myself).
If I'm in a social setting then I just walk right up and say hello, introduce myself, shake hands. I figure that's the reason we're there, guys will expect it, and especially if we're wearing those "Hello! My name is..." tags. If you wanna meet guys then getting involved with LGBT groups that have social functions (political, charitable, church, arts, etc) makes it easy, where introductions are the purpose.