I use GDP only a benchmark for how ridiculously large sums of money we are talking about here - I realise there is no link between the two.
GDP seems a bit useless as a comparison.
You would probably have better luck:
Picking your currency (to convert everything to).
Determine the total monetary supply per country of state fiat (physical and virtual).
Determine the value of valueable products: gold, silver, diamond, platinum, copper, coal, gas/oil (non crude).
The current market cap of Bitcoin
The value of all the land and properties (houses, hotels, ships)...
Your total will give an approximate to the maximum value of Bitcoin but may take a few years to gather.
It's also worth noting that 1 sat could equal $1 due to hyperinflation at some point.