Just pawn the gold off in the muggle world, and exchange the cash for wizarding currency. You'll probably make a lot more. Or better yet, find mundungus fletcher, have him put a confundus on a bunch of cheap crap and charm it gold, then pawn it off all over london.
HP magic is fucking broken.
Polyjuice while selling it, and by the time the charms have worn off and people realise it's fake, the trail will have gone stone cold. As long as you don't repeat it, nobody will have a clue.
Or better yet, get some copper pipe and Gemino the fuck out of it before offloading it. It might start to go to rust after a while, but you will be LONG gone by the time that happens.
Their are Sooooo many ways to make money in this setting it's un-be-fucking-leavable.
Got a fifty pound note? Gemino. Buy a chocolate bar from a corner shop and get two twenties, a five and change in clean money and it should be months before it starts to dissolve but by then they would have used, and that person would have used it and so on. Spend the change going about your day... then gemino the original fifty again and just go to the corner shop down the road. (I would suggest using those self service machines in tesco and the like, but they weren't around then, I don't think)