By counterfeiting the stamp, gold from these places can be passed off as being from one of the acceptable regions of the world. This is effectively smuggling items through a metaphorical blockade.
Say you've got a source of gold that no one else will touch (illegal mine, sanctioned country, slave labour, stolen gold, etc). It costs you way below market price, but no one will buy it from you.
You pour it into bars, stamp it with fake brands and sell it at market price to anyone you want.
Instant profit and your hands/money are clean.
https://www.hrw.org/news/2016/04/12/dispatches-child-labor-d...
Gold may be the dirtiest commodity out there. There are plenty of people that buys gold without worrying about its origins, to immediately melt it into something else. People that doesn't care where a meth head got this 24k gold necklace, will trade it for about 10% its value and melt it into jewelry again.
I'm sure the last worry of people buying gold bars is the source of them, specially if you can get them at a small discount. "Hey, look: I'm getting rich selling tobacco to children, weapons to both sides of a war, paying doctors to prescribe my drugs, flooding the oceans with crude oil... but that gold origin is what worries me at the end of the day."
A majority of our gold testing was designed for jewelry and used a rough glass square and an acid blend for the different major carats of gold we would see. It worked by rubbing off a small part of the gold onto the glass then adding some acid and watching it's reaction, if it wasn't gold or was lower purity it would dissolve and disappear quickly. It was rudimentary but it worked well enough, there were some subtleties with exactly how it should react that you learned over time.