whasian: I'm currently installing the 33Gb Witcher 3 installation, but on the downloads page there's another Witcher 3 download that's 22 Gb in size, what is it and do I need to install it?
I guess that you are using GOG Galaxy (optional) to install the game.
At the moment you have 3 ways to download a game from GOG:
1. Use Galaxy (download is unpacked and because of that bigger).
2. Use your browser (Account - Library - Game - files are on the left side of the pop-up) - smaller files because it is packed.
3. Use GOG-Downloader (packed)
So just decide for one way. But let me say this: If you are using Galaxy you only get install files for a later install/reinstall of the game if you are using the backup function (more button). So you get the same files as using number 2 or 3.
