Interesting question. If we dispensed with all money and were reduced to bartering then the GNP would probably not be affected. After all, money is nothing more than a convenient substitute for bartering – that’s how it came into existence.
Historically all trading was done through the process of bartering, but this has inherent problems. What would happen if you had a cow to trade and were looking to acquire some fish, doing a straight swap wouldn’t be a good deal as you’d need an awful lot of fish to equal the value of one cow. You could balance the deal by cutting the cow up and trading just a part of the cow for the fish – but then you’d have a dead cow.
So instead the idea of a nominal currency was born and throughout history all sorts of things have been used as currency such as animal hides, salt (that’s where the word salary comes from) and corn. Now it’s much easier to trade. You can offer the fish-seller one animal hide in return for the fish, he can then buy your cow for ten hides. After the bartering is done, you have your fish and nine more hides than you started with - which you can then exchange for other goods.
In order to avoid dragging sacks of corn or piles of animal hides around (or whatever the commodity was), tokens were introduced, these were the first coins; one coin for example might be worth one sack of corn and whoever had the coin could exchange it for good and services to the value of a sack of corn.
In time some lucky people amassed huge piles of these coins and it wasn’t practical for them to carry them around. Instead they would lodge the coins with a trustworthy person who would write a note saying that they were holding X number of coins and would return them on production of the note. The note itself then became currency and could be given in return for goods and services, the bearer of the note knowing that they could exchange it for the coins whenever they wanted. It’s from these humble beginnings that we have the current system of banknotes.
In fact, money itself has no value per-se. A banknote is nothing more than a promissory note – a kind of IOU. It’s a promise that the issuer makes that allows the holder of the note to demand payment (as per the face value) when the note is surrendered.
Today many currencies are backed by gold and so, in effect, the banknote in your pocket is a promise to give you an amount of gold equivalent to the value of the note as and when you demand it.
Essentially then, when you buy something with money you’re actually bartering, albeit in a convenient form. There’s actually nothing to stop you renegotiating the value of the money you have. You could for example demand realisation of your banknotes and the issuer (normally a bank) would offer you an equivalent amount of gold, you could reject their offer and demand more gold; but given that they’d be working to a fixed exchange rate you’d be unlikely to succeed.
EDIT: As per the reference to the “immediately preceding question in this category”, I’ve previously answered that so won’t reiterate the details here.