Saw many times for few last years reports ppl cant alttab or minimize game. Very random games.

What OS you have? Starting from WinNT XP Ctrl+Alt+Del open Tesk Manager on top by default or Windows security screen forcely minimize everything. Its just minimize whole Desktop.
Then it fome to WinNT Vista and WinNT 7 where minimaze Desktop is default behaviour of Ctrl Alt Del. There is also mutltiple Desktop sentences introduced in Win7.

There is no way you cant do anything when any game frozen. Windows 8 no more NT but backported mobile OS. It have forcely windowed modes. Native rullscreen mode no more native fullscreen - just emulated. Differ emulation for DX11+ games, DX10 and DX pre-9.

Win8+ alttab, win+esc ctrl alt del - they all always work. After all there is Win+Tab to switch Desktops if you on Win7 or Win8+.

Also. Alt+F4 or power button can be set to open shutdown MENU which one also minimaze the whole Desktop and forcely on top. Not like Task Manager on-top mode but minimize everything at same time.
You can Cancel it and probably your Desktop screen will appear. Or Task Manager. This way not always Desktop reapper.

Looks like something very wrong with OS.

First try to uninstall junk applications with overlays. Etart from MSI Afterburner and RivaTuner RTSS (reinstal RivaTuner from its own official installer, not from MSI).
Close - make sure it closed(*) - Nahimic, NV Experience, some Keyboard/mouse/gamepad apps like LogoOptions. Anything fome with drivers but NOT real part of it.
Or maybe unistal them too. They NOT a drivers and ALWAYS make problems. Eome did not even appear in the game but they work as overlay anyway.
Discord app too.

Reinstall game +verify if you use Galaxy.

*Nowadays Windows as mobile backported work as mobile OS. Closed apps not close for real. Use Task Manager. At least main tab. Better - check also detail tab.

Check Compatibility tab after reinstall. There is fullscreen optimisation disabled option should he enabled by default by GOG installer/Galaxy scripts. Better keep it on. But try to experiment.
Do not check on other checkboxes!
Only this one. And with DPI options on additional window near first one option.

GOG also put additional compatibilities stored deeper in REGEDIT and SYSFOLDER. Should be ok with it.

There is DisciplesGL Wrapper-mod. For this one disable ALL compatibilities enabled by GOG. Better make a copy of game exe and rename it to Game1.exe. Use new copy to laucnh the game with DisciplesGL.

Or ty Narsoul's DDRAW Compat wrapper 3.0.1 (for Win8+, 2.0.1 for Win7). Again - EVERY COMPATIBILITIES SHOULD BE DISABLED or rename game exe to Game1.exe.