Wo-o-ow... khm...

TLDR - reinstall everything to X:\Games\ . In future, avoid using C: drive for gaming\games.

Redownload GOG galaxy installer, Remove Galaxy and GOG Games entirely, clean folders and WinRegisrty from it.
Create folder D:\Games\GOG\Galaxy\
and X:\Games\
run new downloaded installer as admin from X\Downloads\ folder (also make sure to uncheck this files downloaded and potentially harmfull from exe file properties) and install it to path above.

Start GOG Galaxy as admin set up it (downloads-temps and games folder to X:\Games\). Relaunch it without admin and check if correct settings are still there. To install game from client you need admin rights again - relaunch client as admin (it not needed later to just play games from Games folder, but for install and games first launch).
Install game from client and verify game files twice-third in a row (cause Galaxy always redownload some redists-configs and not tell you which ones).
Start game directly from Galaxy (cause you run it as admin before) to finish installation.
Relaunch then without admin rights.

Now game is DRM-free and not need admin rights, if it still in \Games\ folder.

This is all only for GOG Galaxy client and GOGGalaxy version build. There is also standard GOG ver builds - compeltely offline installers.
You also can add them to Galaxy, with soemthign similar in instruction.

Or you can skip Galaxy entirely. Just redownlaod offline isntallers and start it as admin (check digital signature is OK in exe file preferencies, do NOT uncheck validation check during installation setup). Install game to X:\Games\The-Elder-Scrolls-5\ -- no system foldrs, drives root, non-latin letters spaces and "_" and any special symbols.
For first time start game as admin, or press launch button when installation finished (as installer been in adminmode - game also started as admin).

Again, when you remove older installations, make sure you delete old game folder, and especially game folder from .\My Documents\.

After first launch (eg from installer) - you can relaunch game without admin and usually it not needed for games.

If you usually use system folders for games - there is a lot of game related fiels in VirtualStore fodler in C:\Users\...\ and somewhere deep in WinRegistry -> HKCU\... .

What version is "The Elder Scrolls V: Skyrim Special Edition"?
1.5.97 or 1.6.XXX?

GOGDB lists it as 1.6.659.0.8

Regardless of whether you like it or not, Nexus Mod Manager has always been pretty bad and nowadays it's not just bad, it's unsupported. Vortex is the most mainstream manager of this day by the same authors, but it's not that better and very restrictive for the sake of noob-friendliness which ends up backfiring and making it cumbersome to use properly, so I'd advise you to use Mod Organizer 2 instead.

GOG is definitely getting better. It's a time of some sort of renewal with all these new releases and general better mantainance, smoother interface, etc.

Aside from the already given above advice of never installing games you intend to mod (or rather any games) in the Program Files folder, there's absolutely no reason to use FNIS in 2022. Use Nemesis instead, and even though old mods made for FNIS work fine with Nemesis (both tools do very similar things, Nemesis just does it better) always prefer newer mods made with and for Nemesis.

That's what I'm talking about!

So if I don't use FNIS, then would I still have problems if I maintain my current installation in the Program Files x86 folder? I intend to install hundreds of mods.

But transferring the game to a new folder and reinstalling all the mods that I had already installed would be a major pain that I'd rather avoid if possible.

It seems like the relatively seamless procedure that I've seen people making guide videos and posts about, for transferring the Steam version and its mods to another location, does not have any equivalent alternative option without using Steam (unless there is an obscure solution that I don't know about)...but it sure would be nice if there was such a thing.

You know, GOG, just when I think you couldn't possibly be any dumber, you go and do something like this... and totally redeem yourself!

Still wanna go to Aspen???

This is big *ss news.
Now by the gods, give us the Dark Souls trilogy too, Dragon's Dogma is waiting for it!

The product description says it features "game version rollback".
Anyone knows if you can switch back to version 1.5.97 (pre AE update)? Many mods still require the SE (SKSE) version.

No you can't.

I use the offline installer. But I already figured out how to edit the .ini file. ;)

Oh, this is a nice release. Thanks, GOG!

I last played Skyrim when the original version came out . Now I just have to wait for the 24GB to download so that I can yell at Lydia again. :)