Mail shows 100% usage for Gmail account, doesn't rebuild
Try this. Quit Mail, then go to your home folder's Library folder (*) and move the folder com.apple.mail from the Containers folder to your desktop. Start Mail up and see what what happens - hopefully it will start behaving again. If it does, trash the folder you moved to the desktop. We've had several instances of odd Mail behaviour and trashing the Mail container has done the trick, restoring normal function.

(*) If you don't know how to access your Library folder, select the Go menu in Finder and hold down option/alt key to reveal the Library option.