I wanted to gift a cute little .99€ game for my GF, but all the payment methods seems to be unavailable. Even wallet credit. How do I go about unlocking some payment methods for gifts?

So this means I will have to buy 10€ of game(s) and then wait 3 months? There is no way around it?

Unfortunately not. You can thank all the scumbags and their credit card fraud for that. =/ Although you could open an account for your girlfriend and buy it some games (?)

As the old saying goes:

"Everyone pays for theft."

Sadly, no. GOG has been burned by thieves and scammers before. Not saying you are, but they can't afford to take the risks and losses other bigger stores can just eat.

I think in the past someone posted saying that they had contacted Support and they had arranged an exception for this rule.

As I understood it, that's only necessary if you want to buy the same game again.
You can buy different games within that period.

Which game(s)? Might just have an extra copy laying around. I'd rather at that point, just buy the game, and download/share the installer; rather than going that round-about route. In the event another sale comes up where it's on sale may buy it again then as a gift if he really wants to.

It won't be a huge loss. Pretty sure brand new accounts were under the 3-6 month wait before they could just purchase gifts.

The only restriction related to buying the same game is 24 hours before you can gift another copy.

That's not true in all situations.

For example, with many/most (possibly all, but I'm not sure) of the Insomnia deals, GOG does not allow the customer to buy any copies of any game that they already own on their GOG account.

Insomnia deals aren't relevant to this thread.

Ah, the guy has a new account ... I thought his post was related to gifting multiple games

Maybe. Since July is fairly new; though it seems a bit muddier than that. So i am not sure anymore.