1. It's Cypherpunk, not cipherpunk.
  2. Blockchains are not inherently useless, but they've been around a long time (pre-computers, easily) and the only useful blockchain in all of history has been the bitcoin blockchain because it's got the majority of the miners on the planet working on its proof of work. All other proof of work, proof of stake, and proof of everything else blockchains are still quite useless in practice. Maybe one day they'll come up with a blockchain that can be secured in another way.
  3. Calculating the 'true GDP' is about as likely as as finding out how much physical cash traded hands planet-wide yesterday. You just can't see the type of transaction on the blockchain to quantify any of it.
So to answer your question; I wouldn't try to estimate it.
But per community in places like El Salvador it would be easier. A much higher percentage of those transactions is for goods and services.