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".

But money orders can be treated as fiat if enough people agree to do it.

https://en.m.wikipedia.org/wiki/Miniassegno

I mean so can, like, grams of lead but that’s not really the discussion we’re having at this moment.

What do you think a money order is?

Maybe stamps.

No there isn't. At least not in the European country I live in.