http://www.gog.com/en/support/websit...oads_and_games
"2. Can I re-download my purchased games? Is there a limit to the number of re-downloads?
You can always re-download games bought at GOG.com via the “My account” page. Also, there is no limit to the number of redownloads, but please remember that you're not allowed to share your GOG.com account with other users as only you are entitled to download games from your account."
"4. Can I make backups of games downloaded from GOG.com?
Yes you can, and as a matter of fact we strongly recommend you do so – backing up your stuff is a very good habit, you know? Because our games are DRM-free, as soon as you download the setup file, you can back it up on a DVD or your external hard drive without hassle. Plus, as our installers are wrapped in nice .exe files, you can save them all in one folder and create a nice local backup of your games library! And if you forgot to backup your purchased games, fear not. You can always redownload them from our website for free – see below for more info. Sure, downloading will take a little bit longer than launching an installer from your backup disc, but that's no big deal."