Wishbone: And how exactly is it that you circumvent that? I've never bought anything on GG, but I must admit this has me tempted. Isn't it something like starting the installation, then finding the installer file somewhere and copying it somewhere else?
1. Download the file from GamersGate [GG] that downloads the relevant game
2. Put this file on your desktop (or elsewhere you want the temporary files) and run it
3. After the game has downloaded it will attempt to run the installer... don't press anything!
4. Go to where you placed the file and there should be a temporary folder that the GG file created. In this folder is a randomly numbered folder. Copy this numbered folder elsewhere and voila, you now have the installer plus relevant files backed up and ready for installation at any time without the need to use the GG file
5. Complete the installation
Hope this helps