What does GDP have to do with market cap?
There is 30 something trillion dollars in offshore bank accounts. How much in physical gold and silver? How about paper gold and silver? USD? EURO? 1m/btc would be easy if we reach universal adoption. Of course, you'll start to measure inflation (or deflation) by a basket of good purchased with btc... and measuring btcs value in fiat currency will be idiotic.
Nothing. Even in the most optimistic case of bitcoin taking over, you can not compute the value. You can not predict how large part of the value each actor has produced, he wants to have in reserve, in the form of value compressed into bitcoins.
You know, given the fact that his compressed value tokens are safe for plunder, an actor might want to have much more than now in reserve, consequently the value of all bitcoins might be larger than the current value of all fiat money.