Many gift cards these days are of the type where they don't become valid until they've been swiped at the sales till and then can have credit added or removed from them in further till transactions.
So if I found some gift cards lying on the ground I'd probably assume that they were either
(a) used cards, where all the credit on them had already been spent ; or
(b) unvalidated (and therefore worthless) cards, nicked from the shop display by some kids, who'd just tossed them aside when they realised they were of no use to them.
Therefore it's likely that I'd just leave them where they were or pick them up and throw them in a bin.(i.e. I'd assume that they were worthless).
Our local police refuse to have anything to do with most lost property
https://www.suffolk.police.uk/contact-us/report-something/lost-and-found-property
and there are only three police stations open to the public in the whole of the county anyway.