Minerva and Griphook weren't surprised by the idea of taking raw gold and turning into coins. Presumably this happens sometimes; it's not the case that all the Galleons in the world were first minted thousands of years ago. So how did the gold in the wizard economy get there in the first place? And what is the mechanism that prevents it now but didn't prevent it then?

Are all wizards in the world unaware that Muggles possess gold at all? Surely not; Muggles probably own much more gold, and operate many more gold mines, than wizards do. If wizards ever went looking for un-mined gold, they'd encounter Muggle competition.

Wizards have an apparently trivial method of acquiring gold: Apparate into a bank vault, fill your Bag of Holding, Apparate away to Gringotts. It's doable by most wizards, carries no real risk, is unnoticeable by the bank, untraceable when they do notice the gold is missing, and the other wizards and goblins probably don't care if some Muggles were robbed by an unknown wizard.