It depends on which GPU we are talking ba.
GPU sag usually happens when the cooler is too heavy and the card is super long.
When i mention long it means over the PCIe slots and over the Chipset heatsink.
As for Fury X and Nano, their card is just nice after the pcie bracket, but stays in the middle of the chipset heatsink. Nano will be just before the chipset heatsink.
Usually short cards, even i added the WC block (which is relatively heavy), they cant sag, cause the overall rigidness of short cards are simply too sturdy.
Cards that are short wont have sagging gpu.
Long gpus will need something to support it. You can add a chop stick with black spray paint and put it inside to support it. It will be fine and stealth.