If a person that says 'glhf' says something bad at the end or blames the game I do not hold poor opinion of them, unless I play them again and they do the same thing. Sometimes we all get frustrated at the game and might vent some rage in a silly manner at an opponent, even though it is not their fault. I understand this and brush it off.
If a person that does not say 'glhf', says nothing during game then explodes into rage in the last minute I regard them as an idiot and a rage quitter. Typically these players leave the game without saying 'gg', unless they win the game. I tend to just send these kind of people smiley faces when they behave like this but now I am beginning to strike back (which is bad) but I am sort of fed up with this behaviour. Hopefully if chat is reintroduced back into replays people might think twice.
You would also be surrprised how many players don't actually say 'glhf' yet will still say 'gg' at the end of the game, whether it is win or loss. Not all hope is lost

As for saying the winner saying 'gg' before the loser I must admit I do this, but not in offensive manner and only if there is like 5 VP's and I know 100% the game will time out then I will say it. Do people actually take offense to this? I notice the majority of time my opponents still say 'gg' in return so I have always assumed people accept it. Likewise, if my opponent is winning like in this situation and says 'gg' I return the gesture.