robynred: I feel like I'm going crazy, I played Underworld 1+2 about a year ago on Mac and I swear I was able to go from fullscreen to windowed mode and back at will, but I can't figure out how to do it. Before you ask, I tried Command + F but it does nothing. Any help would be appreciated.
GOG's DOS games for Mac used to be run on Boxer. However, boxer has 32-bit elements and is no longer compatible with macOS 10.15 Catalina and will not run on that OS.
Therefore, GOG is updating their DOS games to run on DOSBox instead, which is Catalina-compatible.
The difference you note is the difference between Boxer and DOSBox.
If you are NOT using Catalina, you can download Boxer yourself separately (
http://boxerapp.com/) and run the games with that instead. You will need to locate the game files within each game's GOG app to add them to Boxer. If you have any trouble with this, let me know and I can help you (it can be hard to find the first time you try, but once you know where to look you should be able to find them for all games).