These are presumably like the novelty fakes yo can buy on eBay. Made from brass, and gold plated, and easily detectable as being underweight or oversize (or both).
If someone were able to produce a credible tungsten coin, then I’d take notice.
The best bet is the Canadian method of making ultra pure gold coins, so pure that they can have a holographic pattern etched on them. A gold content of .9999 millesimal fineness (24 carats), with some special issues .99999 fine.
Then something I would highly recommend is for each bar to be sealed in an encrypted security container that could only be opened with an authentication code provided by the mint, in a certified location. If it was otherwise opened, the gold would no longer be certified until assayed.
Perhaps the precious metal producers should create a new market:
gold covered tungsten bars.