Hello,
just thought I should add my experience with Monzo in Gibraltar earlier this year.
Even though the Gibraltan pound is = to the British pound, I found that Barclays would actually charge for withdrawals and also for chip and pin purchases (3% I recall, this could be wrong). So I used good old Monzo instead :).
Just thought it might be useful to someone!
They’ve charged you for an international transaction. It’s pretty standard tbh. That’s why monzo is so good
Gibraltan pound and Falklands Islands pounds etc may be tied at an exchange rate of 1:1 but they are still separate currencies so this is to be expected from big banks. Many foreign currency exchange bureau also gave different rates for Guernsey, Jersey and Isle of Man currencies too, though like Gibraltar pounds they are also at a 1:1 parity.
Ah I see @Jkb114 and @anon44204028! I was under the naive impression at the time, that it would be business as usual. The more you know, and hopefully this will help others 
Ive been travelling to Gib for the past 10 years or so and never been charged.
No bank should charge you for withdrawing sterling in Gibraltar, and i never had this with Citi, Nationwide or Natwest, so not sure why Barclays charge you 3% although the Barclays machine up Main Street never used to accept UK Barclays debit cards of my colleagues.
If in Gib i would always suggest using the cash machine in Ocean Village or at Morrisons (yep Gib has a proper Morrisons) or the natwest on Line Wall Road or bottom of Main Street 
Glad to hear Monzo just works
Transactions here in Gibraltar are processed in GBP not GIP, therefore no UK bank should be charging you for any transaction made in Gibraltar.
I can confirm NatWest, Nationwide, TSB and Monzo do not charge fees here.
If you have GIP cash when you are leaving, try to swap it to GBP (for the reasons @anon44204028 mentioned above). GIP coins are the same size as GBP coins. Vending machines should take them in the UK.
Yep machines are a great way to dispose of non-UK pounds. Not yet if when the UK bring out the new shape pound coin next year if channel isles, isle of man, and gibraltar are re-minting that denomination to match
Heading off to Gibraltar next week for some much-needed R&R.
It sounds like it all works very simply (the Gibraltar pound sounds interesting!), but happy to investigate weird use cases if there’s anything you’d like a report back on!