So I preloaded witcher 3 off gog's library, and i installed the game with the first setup file that has an icon, the whole "witcher disc" has 24.5 gb in the folder that was made by the install wizard, and 20 something gb of setup files. I am wondering, can i delete the setup files since i have the folder that the setup wizard made? or do i have to keep my setup files that makes 40 gb of combined space with the installed folder?

You can delete the installer and the .bin files. Those are just used to install the game.

Short answer: you can delete setup files.

But I would wait for the release day. You will have to get the last (small) piece of the game to make it playable. I think the best way for now is via Galaxy as it has auto-update feature. But it is still beta and you never know what may go wrong with it. In case it wiped out all the files (not likely, but still), you'll have your setup files (or backup installer) to reinstall the game. I'm pretty sure there will be a way to get this small release day update in some other way as CDPR promised to issue first day patch for everyone, so I reckon using Galaxy is not mandatory.

Whatever you choose to do in between after you've installed and launched the game at least once, you can delete setup files. But as it's a bulky installer I'd keep it on some external drive just to feel safe. Better re-download the installer after the release as it should contain this missing part (though it's only my assumption).

I would never delete the files :-)

Really - I did not take too long to DL them, but it would be pure waste to delete them, provided you have some backup medium (USC disk).

I always store such things, if possible. So I can easily install the game elsewhere, for example on the second PC in my country house, without DLing again. Also you can reinstall every time and it takes 5-10 minutes, with or without internet access. I will delete this version if for some reason I download a newer full version, but not before...