But I thought you sent your Mtgox BTC to Josh and Josh also holds all the real BTC. The Mtgox BTC and real BTC stays with him until you withdraw like an exchange.
I think you're right, after reading his FAQ. I do not understand Gox (never having used it) so it is not surprising that I do not understand how he can hold the goxcoin. Do they have a c2c xfer system?
Bitcoin-withdrawals to a Gox-deposit-address work normally. Gox detects one of their own addresses in the withdrawal request and updates the balance in their database, keeping the transaction off-chain.
So for the bitcoinbuilder site, you withdraw your GoxBTC to a deposit-address belonging to bitcoinbuilder, after which it's credited to your bitcoinbuilder account. Withdrawals work the other way around, bitcoinbuilder withdraws to your Gox-deposit-address.
RealBTC deposits and withdrawals work in the normal way.