GDP is how much a nation produces in a year. Being a billionaire is about wealth. So you're trying to compare two different things - it's like asking whether the distance from New York to Chicago is faster than a Porsche.

Probably the question you're getting at is: could a billionaire buy all the goods/services of an entire nation?

To which the answer is "almost certainly not". With the exception of unusual microstates, the value of real property is those nations easily exceeds the liquid assets of any billionaire because as you start buying up that property, the cost of the remaining property explodes in value.