This is obviously (from previous discussions here and elsewhere) one that has many different opinions. Mine is this: Don't be a jerk, and give others the benefit of the doubt as much as possible.
Some people just say "gg" every game because that is what they do. Some people don't say anything. Sometimes people don't feel like sticking around to converse, or even to wait for a simple response and they just leave. Perhaps sometimes they are frustrated and just want to rage-close the game. Whatever the reason is, I try to assume that they aren't attempting to offend me.
I try to say gg when I lose, or in response to my opponent, but sometimes I close the game in disgust at the shuffle luck that I made for myself.
Some people are just jerks and like to tell you how lucky you got. Those people don't have any doubt to benefit from, so I think saving your anger for them is better than people who don't say gg, or don't wait for you to do so.