GBS Share definition Flag GBS stands for Greasy Bum Sex. This happens around most of the world and people never want to admit to doing it. Mainly in Mexico Katie: I love GBS Matt: SO DO I Katie: lets have it now Matt: ok Katie: i smell GBS by Bellymished February 2, 2008 0 0 Get merch Get the GBS mug. Buy Now — $32.95 See more merch