Time legitimizes things too. Leave some to your grandkids in the will. Tell them to wait at least a decade before cashing anything in and then to not do it all at once. A decade or two and they’ll try to sell it, show the will as proof they own the gold. Where did grandpa himself get it from? Who the fuck knows, not like he gave us documents and it’s been too long to ask anyone who might know.

Unless the tax authority directly investigates then, they should be good.