I currently have MM6&7 on GoG and MM8 on disc, playing on windows 10. I can get the games to "work" fullscreen with the exception of several bits of the HUD cut off - as in several IMPORTANT bits. All I ask of GoG is to get this shit fixed up so I can play these games.
i dont have this problem. runs fine full screen here, win10 nvidia 980ti.
maybe its specific to your card..or something..but not a general problem with win 10. just saying they probably havent fixed it because they arent aware of it. hope you get it working tho.
Edit: runing it at 1080p. dunno what res you are trying to run it at