Using the GrayFace patch will allow the game to launch but I thought it may be helpful for folks to know how to run the vanilla GOG install on Windows 10.

By default the GOG installer sets these compatibility flags in the Windows registry:

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers]
"C:\\GOG Games\\Might and Magic 6\\mm6.exe"="~HIGHDPIAWARE"

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers]
"C:\\GOG Games\\Might and Magic 6\\mm6.exe"="~HIGHDPIAWARE"

But trying to run the game with these flags will result in a launch failure with an error window stating:
DDraw-> SetDisplayMode() Failed

To set the correct app compatibility flags, copy the following text block into a text file with the file extenstion .reg:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers]
"C:\\GOG Games\\Might and Magic 6\\mm6.exe"="~ DWM8And16BitMitigation HIGHDPIAWARE"

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers]
"C:\\GOG Games\\Might and Magic 6\\mm6.exe"="$ DWM8And16BitMitigation HIGHDPIAWARE"

Double-click the text file to import it into the Windows registry. This will allow the vanilla GOG install to run in Windows 10. Again, I know that GrayFace's patch renders this moot, but it's just a FYI for those who are interested in such things.

Sorry for large text. This some parts for topic srter and some parts just general info for anyone new to GOG\old games.

GOG always put HIGHDPIWARE to every games not matter if it needed and what OS used. Better if they do admin run (as they do previously) and DISABLEDWM + DISABLETHEMES (sometimes they do disabledwm).

But they also sometimes set additional compatibility with Microsoft ACT sdb packages installable to system (cannot be just edited in registy) (use "detect safedisc", "disablealttab" and something else - that everything also possible by registry entry, but Application Compatibility Toolkit which one listed everything compatibility possible do them to sdb package).

And there is should be sdb package with more compatibilities set, by GOG. Cant see it for Galaxy build, but it is left from GOG ver. in Steam version in 2014.

Or GOG DirectX (ddraw) wrapper (now with dinput).

And this 8and16bitmitigation as well sometimes. Directly by registry or from sdb package.

But this game have very old GOG Galaxy build. Nothing of this is used. Isnt it offline installer way more updated? GOG Galaxy builds is superior for GOG currently, offline get updates without updates (just galaxy compatibility files to get gog galaxy game downloader <=> offline installer and possible to add offline to Galaxy as Galaxy build). And yet they always differ - sometimes even offline are more updated and more content (by mistake).

Of course, using solution without 3rd party downloads is always better. Thanks to find this...
...
But there is two thing:
is "~" and "$" are needed symbols? For me looks they not. Never see them in my registry for all GOG, Microsoft ACT and mine manually set ups.

Another one:
On Windows 8 and Windows 10 first u need to set - and GOG sometimes also apply this (but sometimes 8and16bit...) -
DXPrimaryEmulation-DisableMaxWindowedMode. This is second.. no, third thing needed to try - after adminrun+disabledwm\themes\+highdpiware (experiment with that, disable first). Only after that as fourth thing to try is 8and16bit... or else compatibility from GOG's sdb.

Needed fixes\compatibility can be described here
[url=http://www.pcgamingwiki.com/wiki/Series:Might_and_Magic]http://www.pcgamingwiki.com/wiki/Series:Might_and_Magic[/url]
As a Wiki it flooded with a lot of wrong solutions (GTA series are worst example, same for Steam Community guides... gosh!), 3rd party downloads and else - instead of some proper way like compatibility proper settings. But usually this is also wrong. Usually recommend 8and16bitmitigation - but it should be DisableMaxWindowedMode.
MnM series+sub-series as well. They need to have this more proper way solutions, like as you post here!

Such extended compatibility started from HIGHDPIWARE should be turned off, when used any downloadable wrappers\fix\mods\etc.

GOG better than Steam cause they always do test game releases (with or without publishers). They do. But on unknown machines\OS. Win7x64 currently not supported by GOG and yet for old games someone can test it on Win7x64 only. It something personal from GOG stuff.
Steam version - although for old games usually based on GOG builds (with some GOG files left) - Valve do not take actions here and it is only publishers... not take responsibility as well, usually. Been based on GOG, can be not tested at all. But i never faced completely broken release on GOG or Steam. Never.
Win7x64 = every games works. Except ones where there is coded block for Win7x64. Obviously Win7 can handle every games except UWP (Ms Store). And i dont think there is some more new API possible to prevent Win7 do this. More UWP and hardcoded blocking only. Newest DirectX are not present, but wneh released Win7 still can support it partially and PC support by hardware. Now manufacters do hardware things "hardcoded blocking". More problems than from Microsoft. Telemetry and mining in our hardware by our money... Cyberpunk that we deserve... do we deserve?

So DisableMaxWindowedMode and 8and16bit... are needed only for Win8\Win10\Win11 (+DirectX9 installed and updated +Legacy DirectPlay enabling ofc).
And this is also for Steam version, right? Or Steam is working as is on Win8-11?

Why i ask? It because of offline installer version. Im too lazy to check, but Steam obviously based on build from GOG a much older (2014) than current offline installer (gfw_high.ico file) (hmm, and even GOG Galaxy build). Steam version is not configure to set compatibilities or install sdb package (and this is possible for Steam, but lazy publishers). And mm6.sdb from GOG still there. But not in Galaxy. Looks like it should be even in older MM-6pack all-in-one installer and some more compatibilities are set from this - initially from GOG.

So it probably GOG Galaxy release are broken\have less content. By mistake. Another example when by mistake offline is superior version. At least for all-in-one build (can be downloaded by direct downlink from GOG API, same as every else bundled games incl Heroes Chronicles 8-pack).

Example link to download older bundled package installers - for Heroes Chronicles (ofc you need bundled games+bundle in library)(2.0.0.38)
https://www.gog.com/downloads/heroes_chronicles_all_chapters/en1installer0
Usually this is not needed. Separate games should be separately installed. Chronicles although is not separate games, so this one is good to download. And if current offline build is missing proper compatibility - MM-6-pack is good to download too.