Its just an etiquette thing. In this game the convention has become to say "good game" after every game to mean "I appreciate you playing with me"
Its the same as saying "how are you?" in English as a greeting - in reality people are rarely actually asking that question - they are just saying hi - and the correct response is "not too bad - and you?".
Certainly disrespect doesnt come into it - its just you are interpretting gg in a literal sense - and others are using in in a nonliteral sense.
(Unless, as your title suggests, you mean folks saying gg before the end of the game. I can see how thats annoying - but if the game is very clearly over I have no issue with it)