Please note that when you install a game via GOG Galaxy, it will first download the game files to a temporary game download folder. By default this folder is located at C:\Program Files (x86)\GalaxyClient\Games\!Downloads\temp, even if you chose a different drive and folder for installing the game.
If you don't have enough free space on your C drive to download the game, you can either free up some space, or change the temporary game download folder. You can find instructions on how to change this folder here.
Please note that when you start a game installation via GOG Galaxy, you can see the DOWNLOAD and the INSTALLED size of the game:
- The DOWNLOAD size is the amount of space you need on the drive the temporary game download folder is located.
- The INSTALLED size is the amount of space you need on the drive the game will be installed to.