...Good lord, the old vs new guard divide must be getting pretty damn wide. I just had a conversation on what "cake" used to be called. I've been here too long. :/
Basically, do you know how "traps" are guys who attract guys by being so effeminate and/or cute? The concept of GAR is the exact opposite of that: Men who are so ABSOLUTELY MANLY/BADASS that it gets typically straight guys would be like, "Shit, I'D DO ANYTHING HE'D ASK!" because he's just that masculine!
The archetypal example used to be Archer from Fate, but nowadays Kamina and Solid Snake are the big ones. Real life examples are rarer, since sexuality gets more in the real world, but Bruce Campbell, Idris Elba, and Fight Club era Brad Pitt can get these reactions.