Whenever I run the game on fullscreen I get a blank (black) screen with only sound, I can't alt+tab or close the game anymore.

If I kill the dosbox process from another tty, I end up with a broken desktop and keep logging into a black screen every time I reboot, I get to start reinstalling packages and whatnot.
Never mind that, the problem is: I get a black screen on fullscreen. Anybody else?

I'ts got to be something in the repackaging GOG does with the game and dosbox, since other things I run from dosbox myself seem to run fine.
I'm on ubuntu 22.04, KDE 5.24.7 X11 (no wayland), with compositor off.

Try this: Go to the game directory and copy the game files (you know, the .EXE, the .GRP, etc) then uninstall the game. Now install DosBOX from your Linux repository (if you didn't), create a DOS directory (if you didn't yet) and put the game files there (as if you isntalled in DOS) then enter DosBOX, mount the DOS directory and test the game.