GDP is super easy to fake. Just sell something on the black market. Just trade a service for a service instead of paying for it.

Or, to push it in the other direction: sell a service back and forth. I sell you a message for 1 billion and then you sell me one for 1 billion. Now we’ve increased gdp by 2 billion (although you need to live in a zero VAT and zero income tax country to do that as a private person and still have it count)