[GMG] Do we get an extra game for every item or for every payment?

I haven't tried recently, but usually you can buy only one copy (per promo) of the 1 pack and the 3 pack mystery bundles
EDIT: Ok, seems I'm wrong :)

From what i can see its only winter sale and its says with every purchase and not every order. But havent tried myself.

I bought 10pcs of the 1pack in 1 transaction. I got 10 free games after.

It's one game for every game you buy, so if you have 10 games in your cart, you will get another 10 games. Source: I've done it many times and now have a mountain of useless spares that no one wants. That said, the mystery pack (and any other pack) counts as one game so you will only get one additional key for it.

During the Black Friday event, you actually had a really good shot at winning something nice like Alien: Isolation, Titanfall or Dragon Age: Inquisition, but they've changed that and all you get now is fucking Three Dead Zed.