The type you posted above does not really work for one reason. The sag is towards the end of the card and the length of the support by the time it reaches the end won't do anything for support.
There is another kind of GPU support like the one below that will work much better. My take is that if your GPU is sagging minimally, leave it. If it is bad, then get it supported.
I see that you are having a Gaming X 1080 Ti. That card seriously does not need a GPU support. It is far smaller and shorter than most 1080 Ti's in the market.