I know there's feelings of insecurity when a GF wants to hang with a guy friend, even if he is gay, but look at it this way..... If they've been friends for years, there's obviously nothing "more than just friends" otherwise she wouldn't even be with you right now. She would be with the guy friend right now, or they would've tried previously and found it wouldn't have worked and they wouldn't be together at all now. The fact that you know he's gay.......come on man. Just do something fun that you know she'd like to do with you while she's with him. See if she gets the hint.
The thing I find unacceptable is when they hang with past boyfriends. My last GF was doing this so I dumped her immediately. Either the past relationship is over or it isn't. If they're still hanging with the past guy, dump her. I find it disrespectful if a girl does that. (If she was with a new boyfriend and she wanted to come back for some Tom, I'd let that happen

)
L8R,
fm2