TheJoe: Well at least refund the gifter upon having your gift declined. I know for a fact Steam doesn't do that, and it should.
AndrewC: You can always contact support here on GOG and tell ask them to either re-direct your gift or maybe give you a discount for its value.
On Steam you can change the recipient of the gift it if hasn't been claimed yet, send it to one of your email addresses and activate it on your account or just gift it to someone else.
Both solutions suck. And on Steam, that didn't happen. I was gifted Psychonauts on Steam which I absent mindedly declined, thinking it was another shitty Update News thing. The dialogue didn't return and the guy who bought it was billed and was unable to transfer the gift.
As for having to go to support for that is just added hassle. The point of your expenditure is to give a game to someone else. Now, I don't know what happens when a gift is declined on GOG (I've never done it, and no one's declined my gifts) but the way I figure it should work is that upon your gift being declined one of two things happen:
a) You're refunded
b) The code remains unused and can be transferred elsewhere
But, as I said, I've never declined nor been declined, so I don't know what does happen.