I think GP means “exists” as in “doesn’t require a third party to complete the value transfer.”
If you give me a $20 bill, I can immediately exchange it for a case of beer at the bodega with no extra steps. If you give me a money order for $20, I need to find someone who is willing and/or able to convert that money order into a $20 bill before I can buy my beer.
Funnily enough, paper currency originated as something akin to money orders in the first place, and merchants started accepting them directly because they were much more convenient to transport and secure than the equivalent amount of coins. That's why the formal term for paper money is "banknotes".
If you give me a $20 bill, I can immediately exchange it for a case of beer at the bodega with no extra steps. If you give me a money order for $20, I need to find someone who is willing and/or able to convert that money order into a $20 bill before I can buy my beer.