looking at the screenshots your left [main] monitor is set to a different size than the right... this is common when you have two different screens and caused by your Gpu driver guessing the line instead of you telling it where the current edge is

in other words nothing to do with Gog... Just take control and manually set the edge

Windows 10 = [windows key] + [arrows]; up, down left and right... assuming your left monitor is the widest I'd push to the left edge and then up

This will get fixed soon, right? its a minor thing but annyoing for me.

This is also happening to me and it's annoying... only happens with this app.

Thought I'd bump this again as I'm getting this issue after installing a new 1440p monitor as my main, and having my old 1080p one as a second.