Is it a free copy per "game" bought, or per "transaction". For example, do I get 3 codes if I buy 3 games at once, or just 1 code?

Thanks for the help

You buy 3 games at once you get 1 code. So it is per transaction.

Even if you already own it?

Yeah. It's a gift code. I bought 3 games (at once) myself from EA-promo and got one key. And I have Witcher in plenty of places. =)

If you buy one game. You get a free copy. If you already own it, you get a giftable copy (cheers GOG!).
If you buy one game. You get a free copy. If you already own it, you get a giftable copy (cheers GOG!).

So basically every time you buy from GOG, you get a free copy or a gift code if you already own it. I bought a few games (incl. the complete Ultima series), and ended up with a copy of Witcher and 3 gift codes.

For each transaction, a free copy. :)