I have a question about GOG Galaxy. It's about the backup feature. On Steam, I can download a game and when I want a backup, I can do so without downloading it again. It looks like this isn't the case for GOG, when I go to "Backup and goodies" and click on the game, it starts downloading the game again. Can't the backup be created from the game files I have already downloaded and installed?

Maybe there's something I'm missing. Could somebody help me out a bit here? I just bought The Witcher 3 GOTY Edition. Downloaded the game for an entire day and now I want to create a backup just in case. I'm also gonna be modding the game so it's a good idea to have a backup, right? Would it be better if I copied the game folder and just kept it in an external hdd?

The whole "copy the install folder" trick works with some games but not others; in general, I wouldn't expect it to work with a lot of big games. This kind of portability is not a huge concern for most AAA game developers. EDIT: I mean, you could try it, but in this case, I wouldn't expect it to work.

The backup installers that you can download from your library are self-contained offline installers, not just zips, so they have to be downloaded. Sorry.
For anything you want to keep a backup of in the future, if you want to minimize re-downloads, I'd recommend that you download the backup installer for a game first and install from that (instead of using Galaxy to install), then just save that installer as your backup. Galaxy should automatically detect the installed game afterwards, and if it doesn't, you can use the "scan and import" feature to get the client to recognize it.

Also, since you're going to be modding The Witcher III: make sure to turn off automatic updates for that game so nothing borks your mods. :)

Thanks for answering my question
I wish GOG's backup system was more like Steam's
This sucks but lesson learned
I appreciate your help

You may copy/archive folder with The Witcher and import it to Galaxy later. To import folder head to the game page in Galaxy, click on the more button (located next to play/install), select Manage installation - > Import folder.... Personally I never had any problems with importing games.
For the future I recommend downloading offline backup game installers like HunchBluntley already suggested.

With nearly all games (all I have installed) you can easily backup the game folder and when later installing Galaxy again, you can import them to Galaxy and all dependencies will be recreated from Galaxy. I use this method to move games from Desktop to Laptop or Tablet and vice versa. Only Galaxy takes its time so you will have to wait an hour or two with alot of games installed ;)

Actually, that feature is very much needed on GOG, given the fact that GOG forces users only to have access to the latest version of the game, even if that version has implemented game-breaking bugs that will never be fixed.

The one semi-workaround to this problem is to install the game with Galaxy and then roll it back to an earlier version that GOG does not make available in any other way. But what about users who want to have a backup copy of the non-broken version (i.e. before the latest patch broke it) of the game? Currently, they have no way to get a non-broken backup copy from GOG. But if they could make a backup copy based on the Galaxy-rolled-back, non-latest version that they have installed, then that would solve that problem.

Alternatively, GOG could just let the users choose for themselves which versions of the game they want to download & install, instead of assuming the most recent one is the best and giving them no other options even when the most recent one is broken.