I got a code for System Shock 2 during the Summer Sale- you got one for visiting GOG Galaxy IIRC.

I never redeemed the code as I had bought System Shock 2 some time previously.

The Gift Code is still in my Orders and Settings page. It says it was gifted to my account address, so can it be redeemed by somebody else?

you can check to see if it's still valid by trying to redeem it , https://www.gog.com/redeem

the key redeem system allows you to see what the key is before you confirm to claim it ,

which also shows the time left on keys that are ony claimable for a limited time

not sure what would show up if you try a key for a game you already own though

Yes, just send the alphanumeric code to the intended recipient. As far as I know, that code doesn't expire (like Vyraexii said, entering it on the Redeem page should tell you when or if it expires).