If you gift over mail, the person doesn't have to be registered on GoG, right? Just need to type in the code and receive the game?
Edit: Also, how long does the code exist?

You need a GOG account to redeem and download the games. And if the code was bought directly on GOG, there's no expiration date.

The person needs to register to redeem the code.

Most codes are valid forever. There are some exceptions though. The codes people got for the free games during the last summer sale (Spelunky, Gabriel Knight, Dreamfall Chapters) will expire at the end of this month for example. There are also bonus codes you could get from some bundles which were only valid for a year. But you can easily check it yourself by entering the code without redeeming it.

Thanks for the info.

When you purchase a game as a gift, the code lasts forever until it is redeemed. It will show "No expiry" or words to that effect. If the person you send it to doesn't redeem it, it still sits in your order history and you could re-gift it to someone else indefinitely until someone redeems it.