An acronym used by players that are extremely good at a game that know that they already won to insult the noobs by giving them an sense that they actually stand a chance. It is commonly used by noobs who think it means "Good luck have fun" But in reality it means "Gigantic Loser Homo Fag"
An accronym, which can alternatively stand for "Good luck, high five." This is a less common but equally valid usage of GLHF. Used as a greeting at the beginning of an online game.