The problem than become, how do you trade gold to something you can use? First of all, I and most people don't have much idea how much a gold or silver coin will be worth, without looking it up. And looking up would be to check the value in money term. But if money is worthless, then how much is gold worth? Besides, me as a simple guy and not a gold expert. How do I tell the difference between a real gold coin and a 'replica' from china. And how do you give back a form of change? I mean buying a chicken with a £/$250 gold coin seams a but problematic...
For that last, consider these
combibars. That's 50g of gold in a bar roughly the size and shape of a credit card, pre-scored into 50 1g pieces. Each piece is worth around $40 at today's prices. The idea is that you break off as many pieces as you need for the transaction. Maybe it doesn't go down to a single chicken, but $40 for groceries isn't bad.
As for not knowing how much it is worth today; well, today you don't need to know. If things do go sideways, then people will always want to trade, and barter can only get you so far. That's when people will figure it out. Gold has been used as currency for millennia. There's a reason for that, and the same reasons make it the first choice if things revert. Plus the very fact of that history gives gold an edge over other currencies. It's kinda the default choice.
The thing about gold is that it's relatively easy to assay. Especially in a standard format, like a coin or a bar. You measure its size accurately with callipers, measure its weight, and then either compare that with known good values or else just figure out the density from scratch. It's hard to fake that because there aren't many other things as dense as gold. It's much easier to assay gold than diamonds or other gems, where you need to be an expert to evaluate the clarity, cut, purity and other factors.
I can envisage a few scenarios. The first is in the early stages, where people don't trust paper money but still hope the crisis will quickly pass. They may accept gold hoping they can sell it once things return to normal. It's better than trying to get them to take your wristwatch as payment. The second is when the crisis has persisted for long enough that new systems arise to cope. That's when shops become equipped with callipers and scales and the knowledge to use them.
Of course it is not for everyone. If you have limited funds to spend on preparedness, it's probably better to spend them on things you can use directly, rather than hope you can buy what you need later. If you do have funds, then keeping it all in a bank might lead to a nasty surprise one day. Keeping it in dollar bills stuffed under a mattress may turn out not to be ideal either. There's an argument for keeping maybe 10% of it in gold.