When people complain to me (or report) about this, from a moderation standpoint I currently think
-
gg at the beginning: perfectly fine, as stone_defender says, it means “let’s have a good game”
-
gg at the end: perfectly fine, it means “that was a good game!”
-
nothing at the beginning: perfectly fine, there are tons of reasons why a player might not (or might not be able) to greet their opponent
-
nothing at the end: ditto. It is not “expecting the loser to gg”
-
gg: during play: not OK please report it. It is not OK to pressure your opponent into ending the game.