As Justin said, "I could not absorb that much money intelligently".

I think a chunk of currency on-par with the US GDP is not a liquid asset. If you could accumulate such a sum, introducing it into the market would probably deflate whatever currency it was introduced as. It would be roughly equivalent to minting it fresh.

The smartest thing to start with might be a system of checks and balances for your own spending. You'd look like a government at that size, so it's probably best to behave like responsible one.