Plan on getting Morrowind from GOG, but I need to know if the overhaul mod by ornithicoper works with GOG Morrowind or only with the Steam version.

No the mod will work just fine with the GoG Version. Technically there is no difference in between the two version for mods.

It works fine. The only issue is after unzipping the files, you need to put them in your main morrowind directory, or else you the installer can't find Morrowind. Other than that, it had no problems working on my install.

sorry, it's solved

Why is that an "issue"?...;)

Indeed, I thought that was how it was supposed to be done anyways.

Can I use both version on one operating system, or does the registry go loko?

Sorry, What do you mean by"both version on one operating system"

I don't know how common it is, but there are those those that have multiple instances of Morrowind on a single computer.

You can install multiple copies of Morrowind on the same OS and use them. You can also have different mod setups for the different versions.

I'm aware of only one issue: the Morrowind launchers use always whatever Morrowind.exe is in the Windows registry instead of the one in the same folder. To make sure the luncher is using the right Morrowind.exe, I recommend temporarily renaming as _Morrowind.exe the ones you're not currently using.