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