Last year Commandos received a patch making it run on modern computers, included Windows 10. Additionally they patch also 1280x720 as a selectable resolution.
Did GOG's version get patched, too?
I can't say for certain as I don't know anything about the steam patch, but GOG probably does not have that patch.
I checked for both Commandos Behind Enemy Lines and Beyond The Call of Duty with a fresh install through Galaxy.
The highest resolution for Commandos 1 that I see is 1024x768 (in-game menu, I didn't see any external config option).
The latest and only entry in the changelog is from March of 2016. It only applies to concept art added to the bonus goodies.
I don't have Windows 10 so I can't speak to how well it runs on that OS.
I hope this helps.