I am playing The Witcher 3 through GOG Galaxy on my Windows 10 machine, and I would really like to store save gamesaves on my G: drive (specifically G:\Documents\The Witcher 3). By default, the games were saving to my OneDrive on the C: drive. I decided to move OneDrive to the G: drive (by deleting the folder and going through the OneDrive setup wizard). However, this did not solve the problem, as The Witcher 3 started saving files in C:\Users\slyry\Documents instead. I looked all over the settings of the game and online forums, and realized that it used the Windows Registry to determine where to save files. So I decided to head over to regedit and modify some registry keys. Specifically, under

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\GOG.com\Games\1207664643

I modified the SAVEGAMEFOLDER value with the data G:\Documents\The Witcher 3\. I did this for the SAVEGAMEFOLDER values for both DLC's I own as well

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\GOG.com\Games\1430743168
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\GOG.com\Games\1441355562

I restarted my machine, started The Witcher 3, and the game still decided to create the C:\Users\slyry\Documents\The Witcher 3 folder and put gamesaves there. Has anyone found a solution to what I'm trying to achieve? Thanks!

Symlinking would probably do it.
If you don't want to muddle around with the commandline I think Gamesave Manager can do it for you using 'sync & link'.

I just moved my entire My Documents to a different partition.

And for people who prefer GUI: Junction Link Magic.

I think (though not 100% sure) that these are the folders that the uninstaller will delete if you choose to delete saved games as well when uninstalling, nothing to do with the game itself. May be mistaken though, but I'd suggest double checking them if you ever decide to uninstall the game without keeping your saves.

Witcher 3 puts saved games in the Documents folder? That's one of the worst possible places to put them. %USERPROFILE%\Saved Games is sensible, especially considering that the game requires at least Windows 7

It's surprising how little the Saved Games folder is actually used. Developers still like to make a bloody mess of the Documents folder or conceal them in %AppData%.

Thanks, I moved all my Documents on the C drive to G:\Documents and this worked:
mklink /J C:\Users\slyry\Documents G:\Documents