Sry for my english, hope you understand it.
So didn't know that this game don't support controllers but it's ok. I found mod for it - don't work. Uninstalling game and reinstall.
Fine i will play with mouse.
So i downloaded Infernity mod. Not starting in fullscreen, just very small window, when alt+enter then i can only use mouse on like 1/4 of screen, can go only up and left with mouse.
So uninstalling game and reinstal.
I check whay mods actually must work, i found Belzebub, starting in window, when changing to fullscreen i see only half of what i should see, checking resolutions and fullscreens on/off everything is so screwed that i can't even see options and reach mouse to it, need to use key arrows but i can't do nothing not seeing those options cause on fullscreen or not i ses only like 20% of left part of game. Now i can't even revert it.
How many times i need to reinstall game, and the f@#$ what mod actually work with this crap.

Have you tried devilutionX 0.4.0 either vanilla or with the QOL mod?

https://github.com/diasurgical/devilutionX/releases
https://github.com/Manuel-K/devilutionX-QOL-patches/releases

If you've tried multiple different mods and they all give you the same kind of display issues then it could be something about your system setup, like drivers.

Anyway, you don't need to keep re-installing the game. Mods like Infernity, Belzebub and DevilutionX only really require the Diablo data file (at least for single player anyway). So with the GOG installation, just copy the below files somewhere and extract your mod of choice (Infernity, Belzebub, DevilutionX, etc.) into the same location. You can then uninstall the GOG installation and not worry about it.

DIABDAT.MPQ
diabloui.dll
SMACKW32.DLL
STANDARD.snp
Storm.dll

DIABDAT.MPQ is definitely required, but the others may or may not be.

And patch_rt.mpq

devilutionX only requires the mpq files. devilution and infernity require all the other files that Korell mentioned as well. No idea what Belzebub requires.

OK i wasted another few hours to check multiple solutions. Devilution kinda works but i got huge black borders on left and right of screen, not sure if its how it works. Id wish it fullscreen. Tested also with 0.4.0-qol. Anyway Infernity do not work at all. Also x-box controller mod (based on devilution) do not work. But as i sad earlier i can eventually live using mouse.
Regarding Belzebub i found some dude on redit having the same problem as me. Doing some tests as he do the only resolution working for me is 800x600. (luckily for him he was able to play a bit higher).
So i am forced to play on my 24'' monitor on 800x600 :( Game look ugly but i could do it. But then the game do not save my progress. Any time i quit game i can't start where i left, i can only start from level 1 with that char (keeping items and gold tho).
So to resume :
Belzebub - unplayable, if i somehow find the way to save progress then anyway only 800x600
Devilution - playable but only on 2/3 of screen, the rest is black borders
Infernity - not working
Controller mod - not working

I'm kinda sad now, well i original game on GoG works tho but i didn't bought it for that.

DevilutionX gives a vanilla gameplay. As the game was originally for a 4:3 screen resolution, even if you use higher screen resolutions it would still pillarbox it on widescreen monitors. I believe it uses your desktop resolution and just scales up the original assets to fill as large a 4:3 aspect ratio as possible. Ooh, what's patch_rt.mpq. I do have one in my original install but never knew I needed it for these other mods. It's a tiny little file, only 580 KB.

It's the file that contains modifications made by the patches. Do you remember that the original behaviour was to leave DIABDAT.MPQ on the CD? It was never modified during patching.

––– Devilution/Infernity only use the original resolution. Devilution aims to be an exact recreation of the original exe.

Right, what is the native resolution of your screen? DevilutionX (which you tested) still renders at the original resolution and then scales the result to the screen resolution keeping the original aspect ration. In principle it should be possible to modify the code to change the rendering area, but I've looked through the code and (almost) everything is hard-coded. The other issue is, if you change the rendering area, everything will be very small unless you you were to add some kind of upscaling algorithm to modify the assets.

Allright i understand that 4:3 ratio. I could play with that borders but Devilution alone is not what i want. I tought it is needed to run Infernity with all those QoL improvements that it provide.
Now starting Infernity it runs with small window. Alt+Enter create some artifacts on screen so i can't even type my char name when creating one. I read that Infernity do not work well with GoG version now. Does any1 happen to run it with GoG version?
Belzebub seems a good mod for me but i can't make it to run properly.
My monitor can go to 1920x1080 if that matters

You cannot combine any of those mods. All replace diablo.exe with their own executable.

Infernity is based on and outdated version Devilution and it probably will not be updated until Devilution is ready. The author has mentioned that he plans to recreate it based on DevilutionX.

At the moment the best choices are DevilutionX that's pretty vanilla (if you disregard the scaling) or DevilutionX-QOL if you want some of QOL improvements (including the item highlighting and the monster health bar from Infernity).

The data files are the same regardless of whether or not it is the GOG version, so you shouldn't have any issues. Just use the files listed above with the Infernity mod.

If you do wish to use Infernity, I wouldn't use Alt-Enter to switch to fullscreen. Instead, either edit the ddraw_settings.ini file, or press ' when in game and change the resolution options there. Just be aware that fullscreen it will stretch the game to fill your widescreen monitor, distorting the image, rather than pillarboxing as DevilutionX does.

Sorry for the late reply but DevilutionX now renderes in your native screen aspect (no stretching, no pillars), the rendering area is also adjustable if you want to have a larger view area and animations are done at native monitor refresh rate instead of 20hz, which makes the game feel much smoother to play.