From the scummVM site
"4.5. Can I run my game full screen?
Yes. You can either start your game using the -f command line parameter, setting the game to Full Screen in the ScummVM Options dialog, or by using the Alt+Enter key combination in-game to switch between full screen and windowed modes."
Presumably the GOG version does this, maybe you could find & remove the -f
Oh, or you could just open Beneath.ini and change "Fullscreen=True" to False if you want it the easy way...
Just tried setting "fullscreen=true" to false, and now I am running into the same issue that nemt had, and ScummVM appears to crash immediately after starting. Using Windows 7. I tried reinstalling and editing the beneath.ini file again, but the same issue occurs after editing the file.
I had been trying the compatibility settings to get the video to appear properly and now with scan lines and odd colors, but to no avail.
Any assistance would be appreciated as the game will not start now.