Prices are mostly shown in both GBP and €
One can pay in GBP, but if change needs to be given it will be in Gibralter Pounds or smalls, being not valid in UK.
Take small GBP notes and Pound coins to shop with.
Also they will happily take Euros.
All Credit Cards are accepted.
The only true savings are on Tobacco Products, Spirits/Liquor, Perfume, Colognes, Makeup etc.
If considering a high end watch or designer jewellery, check UK retail prices before even discussing with store, as it could be the UK is better priced.
Additional tips.
Purchase all standard brands of Alcohol from Stores in the Cruise terminal, they give a receipt/ticket and one collects on on return. Prices identical to their town store. No need to carry around/from town.
Also, one Cruise Terminal Store (can’t remember which one - you’ll discover) will give a genuine discount Voucher, valid for a particular Cosmetic/Perfume store in town, if of interest.
PS
If seeking special bottles albeit Spirits, Wine, Champagne etc., then drop into VINOPOLIS, and take a look. Sommeliers from Cunard do actually shop with them.