Hi,

After trying to troubleshoot this game for ages I finally managed to find something that worked. For convenience I am using Steam to launch the game but you don't need to as long as you have a recent version of Proton and gamescope installed.

- Add Cossacks3 as a non-Steam game
- Set target to: cossacks.exe
- Set the "start in" field to: /home/<your user>/.wine/drive_c/GOG Games/Cossacks 3/
- Set launch options to: gamescope -W 3840 -H 2160 -w 1920 -h 1080 -f -- %command%

You'll need to adjust the resolution according to your setup. I'm using a 28-inch monitor at 3840x2160 and so far I've only been able to get it working at 1080p. There is a severe graphical glitch when a match begins however it's easily fixed by going into the game menu settings and changing Texture Filtering from 16x to something else and then back to 16x. After that the graphics are fine and the game seems to look and play well. Hope this helps someone.

System specs:
Arch Linux - 6.15.2-arch1-1
NVIDIA driver 575.57.08
Ryzen 9 5950X
Nvidia RTX 3060 Ti 8GB
32 GB RAM