Web slang for the phrase "Good Times Had By All." Used in expressing the good times had by a group of 4 or more individuals
Dude 1: Lol, remember when the 6 of us got on that golf cart and we did a wheelie in front of Inn-N-Out Burger?
Dude 2: Lol, yeah, GTHBA!
Dude 1: GTHBA indeed!