Similar PC Specs here.
I never change the paste and I'll never will on my Asus Strix. In idle it is working at 38ºC (my case is open now since I'm doing some work with it changing stuff lately, it sits at 34/35ºC with the case closed, fans in the GPU not working at all of course) and while playing it gets up to 71ºC ( during the summer) depending on the game of course, but average temp is 67ºC more or less while playing any game. Which to me is perfectly normal. (this card is silent, you can't even tell it is working)
You shouldn't be worried about those temps at all, perhaps you applied to much paste or something but still if the GPU is not working, 40ºC is not going to do anything to that card at all. Judging by the temps while playing you are more than safe.
An engineer told me one thing once "if it works, don't change it or touch it at all, leave it as it is".
