I'm looking for solution regarding "cutted' screen for older games (mostly from GOG, like Battle Isle: Incubation, Alpha Centauri etc), by some reason even dgvoodoo (which helped me A LOT on GPD Win 1 & 2) is not helping.

Tried with dxwnd, also no luck. I've noticed that there is some weird selection of screen resolution, aside from default (1280x800) there are only "reversed" lower resolution (instead of 800x600 there is 600x800, instead of 640x480 there is 480x640), don't know if it's the source of problem, but can't solve it (I don't see anywhere option to "unlock" not supported screen resolutions).

There is some "GPD Tool" preinstalled, which is saying that "turning off 3d acceleration" should help, but when I'm using this option, it doesn't help at all (in fact don't know if it's even working).

I will be extremely grateful for help, person who will provide the working solution, will receive from me the gift with Dishonored: Definitive Edition.

Been a long time since I looked at dxwnd, and don’t have a or win max. But can you not enter your own resolutions for the window? Just had a quick search:
https://nitrotech.info/force-window-mode-games-dxwnd/
Don’t thinks that’s too current, but in the setup you can enter window size.
Just a thought.

Sadly, I've tried it already (in fact done it also in the past on my GPD Win 1), but DxWnd is not forcing games I've tried to run in windowed mode (or in fact "forcing them" to make any changes in terms of display)

Don’t know then, perhaps you can try one of the other options:
https://www.pcgamingwiki.com/wiki/Glossary:Borderless_fullscreen_windowed
Which probably won’t work with those games, or possibly force a resolution in your graphics settings:
https://www.pcgamingwiki.com/wiki/Glossary:Custom_resolution

I always had issues with various games on my win 2 due to the odd screen setup.

Oh, and what happens if you connect to an external monitor?

Games with a .cfg file can manually be set the whatever resolution you set it to. Most games have a configuration file. Annoying to do this outside the game, but it is possible. Plus you can plug the pc into any hdmi screen to play games on a larger tv or pc monitor at a normal resolution.

Let me know if hdmi to a screen works. I wanted to get a 2021 model.

I finally found the solution!

Turns out that GPD has discord channel, where one of users has pinned the bat file, allowing to add any screen resolution to display settings (those visible after clicking with RBM on desktop). I've added such resolutions like 640x480, 800x600 and 1024x768, and if I set it before launching the game (tried on Battle Isle: Incubation and Emperor of the Fading Suns)

The one thing which I dislike is the fact, that the text and some elements seem to be a bit "blurry", but I'll try to find the solution on this as well.