Besides being slotted in, the end of my GPU is connected to the back of the case on both sides with screws. Isn’t that enough to hold it? 🤔
Larger/longer GPUs will often show some sagging when mounted directly to the PCIe slot on the motherboard. In those cases, using some sort of bracket/stand to keep the GPU level not only relieves stress on the slot/motherboard, but also keeps the GPUs PCB from flexing and potentially being damaged as well. If you have a shorter, lighter GPU in your system, then you may not need to counter any sagging.