What’s a simple way to fix GPU sag?

A GPU support bracket is the neatest option. Pick one that fits your case and adjust it so it supports the card without pushing it upward hard.

You can use a small, stable object as a support if it fits. Check that it cannot shift around and stays clear of the fans and other moving parts.

Before adding anything, check that the card is fully seated and its screws are secure. If the sag appeared after cleaning, reseating it may help, but don’t force the card or connectors.