Subject header says it all.

Thank you for the very quick reply

Does it work with multiplayer aswell? And to remind me does it have the full questline with belzebub?

GOG version is the same as the original diablo installed from CD. It will work with any mod that works with Diablo 1.09.

Quests depend on the version that you are playing. Multiplayer Diablo only has four quests: Butcher, Skeleton King, Lazarus and Diablo.

I cant get it to work properly. It runs in a small window mode and any attempt to Fullscreen it does not work.

Have you gone beyond the main menu? You have to set the resolution and fullscreen setting during gameplay itself. It cannot be set prior to starting up a game.

Create a character, load into Tristram, then hit ESC and go to video settings.