MGS GZ for 7 €?

they do and they are out of stock. I got my key - bought the game at start of the promo.

Huh? Mine didn't say anything about being locked. Some games on the site have in bright red that it's locked to South America, and when I tested and tried to buy I couldn't. But this one doesn't have the warning and let me purchase. They won't retroactively lock keys, will they?

Depends on what you do with it really. It's a tech demo/proof of concept disguised as a budget-priced prologue to MGS 5, and the point is to prove the open world mechanics.
You can just rush through, and see the end very quickly (around 10 minutes if you really try to rush it) and be done with it, or you can spend a lot more time exploring the map, finding the side missions, replaying with different approaches etc.