KyleCS: Seems that doesn't work with a freebie game? No codes are shown in "Gifts", even though I'd like to gift to someone else the Duke Nukem 3D I got for free.
Nope, it works for any purchase, free or not. Now, if the game is actually on your library
shelf, i.e. on your account, then you can't really gift it. You could, in theory, pester support to remove it and give you a giftable copy, but a) you really ought to have a copy of Duke3D and play it, it's a GOG thing; b) pestering support might be too much hassle; c) most people already have the game anyway, so finding a recipient might be harder than you think.