If you buy one (ore more), make sure to check the "gift this order" so that you can at least gift (or trade) unwanted games.
I bought 3 and got:
- Dust: An Elysian Tail (already had it on Steam and on Humble Bundle; contacted GOG support to see if they can convert it to a GOG gift code).
- Eador: Masters of the Broken World
- Outlast
I bought one and thankfully (kind of, since it's not particularly interesting game to me) it was something I don't already have, but they automatically make it a gift code if you have it ON GOG already. If you don't have it on GoG but might have games elsewhere then it definitely does make sense to do the order as a gift for everything since you can self-redeem anyway.
Also it says that they try and go for games you don't already have (which is kind of weird, you would think they could force that to be the case, but oh well), so you should get something you don't have, but if you do get something you already own on GoG you get a gift code even if you didn't select "gift this order".