what "gg" means? Good game?

why most of the ppl not using this abbreviation appropriately and accordingly?

i kill you - ppl say "i gg you" (i good game you?)
when there's leaver - ppl say "GG", "gg.com", "gg lar" etc (Is that mean got leaver = good game?)

when enemies get strong heroes, ppl say "gg liao la", etc. (enemy got good hero is challenging, therefore a good game?)