With Gamersgate you download the file and you're given the installation key so I can't see what the issue would be with backing up your purchase and making a note of your CD key.
On gog.com they remove some games after a certain period from their catalogue, but they still have it available to download for those folks who've purchased it.
I'd strongly suggest making backups though in case someone "pulls an Amazon
" though! That's some stupid shit right there.
May your terror missions always be infested with Chrysalids.