I think you are going a bit down the wrong track, trying to suddenly move any amount of commodities or even different nations fiat currency at a moments notice into something else will attract all kinds of attention from taxing authorities (government).
Very few "individuals" ever need to (& have the ability to) suddenly dump hundreds of thousands or millions of dollars of gold at one go. That is usually reserved for corporations, institutions & governments which do it all the time & it's a well established worldwide business.
There are a lot of large brokers that will be able to move millions of dollars of gold quickly for whatever fiat currency you wish.
What price you get depends a lot on what you have.
If it's just bulk gold, then it's going to probably be less than spot price because it has to be tested & verified.
If it's legal tender minted coins in mint condition, you'll get over spot gold price but less than selling price for those same coins.
A lot of the better places will list if you ask both the buy and the sell price of the same item.
If you are looking to move a couple thousand dollars, there are lots of smaller dealers that will buy them from you in cash no questions asked, but do be aware that if you go to the $10k mark or transactions nearing that in total, then there are Federal tax reporting requirements.
If you have good condition minted currency coins, you can always get spot or above unless you are an idiot. It just depends what you negotiate.
Minted coins are usually a bit above spot price on selling, as well as buying so what you get back will be between you and the party you sell to come to terms with & different coins command different premiums.
You don't want however to be trying to sell your gold in the middle of a crash when everybody else is having to sell hard assets to cover margin calls.
