If the other post is to believed, they have around 740,000 BTC. There are roughly 12,400,000 coins total at the moment. That pegs Mt. Gox's share at 6%.
I'm sure a lot more than 6% of all total bitcoins are lost to dead wallets and such already, though. Only drives up the scarcity of the rest of em.
I highly doubt Gox's coins are "gone" though. Someone has those wallets and almost certainly is going to make a pretty penny fucking over the customer base, but caveat emptor.
