Stands for "Gay is not a synonym for shitty" This was originally a quote by Pete Wentz, bassist from Fall Out boy and got turned into a song. Used when someone uses gay to describe something shitty, though it really means homosexual.
Person 1:"That movie was so gay."
Person 2:"G.I.N.A.S.F.S. dude."
Person 1:"That song is so gay."
Person 2:"Don't say that because G.I.N.A.S.F.S.: Gay is not a synonym for shitty."