Gar is an American-made term that counters the concept of "moe." A character who is gar (or who someone else claims to be "gar for") is a character of such intense masculinity that he (or, occasionally, she) inspires feelings of worship.
"Gar" is a term that was developed amongst English-speaking fans to refer to characters who are intensely masculine and/or macho. Probably the first example that comes to most fans' minds these days is Gurren Lagann's Kamina, although Fist of the North Star's Kenshiro is surely a close second.
The term "gar" is generally credited to the image board 4chan, upon which someone reportedly attempted to say that they were "Gay for Archer" (as in Fate/Stay Night's Archer) but instead typed "Gar for Archer."