I previously had GBP set as my currency but after checking my wallet, where it said it wasn't supported, I can now only select USD and EUR. Is there any way to get my native currency back?

I am based in the UK.

Hm, did you maybe change the region settings in your Windows?

No.

For some reason GOG thinks you're not in the UK (Windows' locale settings shouldn't matter). Log out and clear your cookies (only those from GOG.com, if your browser supports that), then log in again. If that doesn't work, contact GOG support.

Thanks, that solved it.

Also make sure if you block localStorage with any browser plugins or whatever that you allow it for GOG, they use both cookies and HTML5 localStorage. This can cause problems potentially if blocked.

Just had a variation of this issue, but I disagree that ZFR's post solves the problem.
It explains what the problem was, and gave me the clue as to why I experienced a similar issue.
Only GOG can solve it, and this is much worse, in so many ways.

In my case I'm using a VPN, and appear to be in Switzerland, but I'm really still in the UK
Regardless of where GOG thinks I am, or indeed, where I actually happen to be at that moment in time.
My preset user settings should override any IP checking GOG is doing/

Plus that raises the question of why the hell is GOG IP checking in the first place.
If you think security, think again, I both auto logged in, and logged out, and manually logged in without GOG verifying my ID.
So GOG's happy with the fact I'm the legitimate account holder, yet ignored my settings.

Assuming I was stealing the account, which I've successfully done.
Without ever triggering the 2 step verification crap, GOG has forced on me, against my will "for My Security".
Then to add insult to injury, they change the Language, and Currency used to the thief's own.

What if I go on holiday to a country, where I don't speak the language, or use their cash.
GOG will force both on me, and the worst part is, I don't even get an option to use the British Pound any more.

So though ZFR gave me an explanation, close enough for me to figure out the problem, it's by no means a solution to this issue.
Plus It's much worse than any Windows locale issue.

My case exposes a serious security hole, in account management.
IP based overrides of previously set user settings, that should never be overridden.
Except by changing those very settings in the normal manner..

Also you should be free to select all available Locales, and Currencies.
Wherever in the world you happen to be at that specific moment.

The region blocking and regional pricing destroyed that and make problems for some of us thus.