I want to buy Saints Row IV but the store page won't let me add this to my cart.

This is probably something super obvious but I can't fix this. I'm logged into my account everything else is working I've had other games in my cart but this one won't go into my cart.

Help please.

The international version of Saints Row IV is banned in Australia (the reason is stupid, ngl). Since GOG doesn't have multiple repos per game like Steam does and doesn't have the adapted censored version, I'm afraid you'll have to try asking a non-Australian buddy to gift it, or use a VPN to appear as if you're coming from somewhere else.

Many games are treated in the same way for German users as well, due to their presence in the infamous BPjS/BPjM list of games "harmful to minors".

Normally you wouldn't even be able to see a game that wasn't available for purchase in your region, but strange things sometimes happen when the site is stressed due to a major sale or similar event.

As the other user hinted at, you'll be able to redeem a gift code for a game, even if you're not allowed to purchase that game yourself. Also, if you are in a different region (or the site thinks you are...), you'll be clear to purchase anything available in that region. In both cases, once the game is in your account, you'll be able to download, install and play it afterwards -- even via Galaxy -- regardless of where GOG thinks you and your computer currently are.

That's not entirely true. GOG has a way to handle this.

Check Kayne & Lynch on gogdb: The same readably game id (not the one in numbers) leads to two different store pages, depending on where you are located at.
Germans can only access the censored version. Everyone else can only access the uncut version.
There are links to both store sites using the number id, but one will appear offline to you (and to gogdb).

They handled Wolfenstein 2 similar, but then they used two different titles for the versions, one called.

It's really up to the publisher if they want to do that.