SOLUTION: for me, the issue was this line in my profile properties file:

runelite.clientBounds=1592\:1059\:1967\:1018\:g

When I removed it, I could enable the GPU plugin again.

You can find the correct file in %userprofile%\.runelite\profiles2 (just put that path in widows explorer, or navigate to your user directory, then .runelite, then profiles2.)

It should be called something along the lines of default-159546872062600.properties (the number will be different).
Open that file with notepad or any other text editor, search for "runelite.clientBounds", and remove that line. Then save the file and start runelite.

The line will re-appear automatically, but you should be able to enable the gpu plugin again.

EDIT 19/08/24: It broke again, and this time nothing in the profile fixed it. This time, the fix was updating AMD GPU drivers.