Just downloaded over 3 GB, which GOG put on the C drive.
I don't use my C drive for storage, so I wanted to move the files to my games drive.
Tried several times and every time it froze up my system and I had to reboot. Very annoying.
Anybody knows a solution for this? Thank you

More detail, please. I take it you mean you're using the downloader and that is what's putting the setup files onto your C drive, right? And you're closing the GOG downloader before you're attempting to move anything, right?

Could have been the GOG downloader still being open.
But even after several reboots?
Anyway, today is a new day and after reading your answer moving the GOG files was no longer a problem.
So, your answer was the solution, or was it the new day? :D
I guess I have to thank both :)

Is your games drive an external drive?

As a proponent of preventative medicine, I'd suggest, first of all, to change the download path ;P.

How old is your system? I had an old laptop with a less than stellar processor that used to choke whenever I looked at GOG executables over 1 gb.

Why not just change the default download path?

I generally use Teracopy to move large files. It does a good job and verifies that the copy was done correctly before deleting the source file.