Why is it real GDP when a government contractor makes a PDF report, but isn't real GDP when I make a poem?

Because money paid to the government contractor results in a person (persons?) getting paid along the way and then a large part of that flows into the real economy via consumption / spending.

In your $50 trillion poem example, no money can possibly flow into the real economy because you simply do not have $50 trillion to pay anyone - you're just describing a wash trade of a worthless $50 trillion IOU note not unlike a NFT or crypto memecoin. Best case is that the poem is worth something non-negative.

That's the point.

GDPNow tracks real GDP.

Ah, thanks! Interesting. How does it know which is Real and which is poems?

Ask DOGE ;)