In the computer game, Diablo 2, gg refers to Godly Gear. Namely, rare items (or glitched/bugged items) that have desirable modifications on them. The specific combination of mods on them that are extremely rare to come by makes an item gg.
gg items 4 hrs (trading godly gear for high runes)
eth bugged armor (double defense bug) is a gg item.