Don't know if this has already been suggested on here but...how come there isn't a way of gifting credit? I wanted to get a friend a game from GOG.com but didn't know which he'd want most out of the catalogue...

Just give him Planescape Torment ;-P.

Maybe Ubivis can shed some light, there were vouchers on the advent calender.

I believe there is. I don't know how to get them, as the only experience I've had is being gifted one after winning a contest. If you want I'd be happy to ask the fellow I got it from how you buy them.

does your friend have an account on www.gogwiki.com? If so, he can upload his Shelf and Wishlist to the Wiki and everyone can access it. Something like a Value Coupon does not exist (yet)

Exist, but not avaible for commoners.

For exapmle, there's CDP loyalty programme in Poland, and for buying their games you may get access for free 5.99 or 9.99 game for points gathered through time. You have to choose what game you want, though. However, It's said that 5.99 or 9.99 code works for any game in the same price range.

But i believe it's changed, since I can't see the option for "redeem promo code" like it was in the past.

I'm not sure if the promotion code vouchers have ended, but the page to redeem them still works.
Its a pity GOG doesn't support a wallet and a wishlist like Steam does and that despite the working functionality, they won't allow us normal folks to purchase promo codes.

They enchance the system and the webpage from time to time, Im sure GOG wallet or game vouchers will be avaible sooner or later. At least I hope so.