GBL is for bitcoin like what Etrade is for US dollars (well, sort of). Imagine one day Etrade just disappeared... Then all the money people had in their Etrade account would disappear with it. If you had your money in your wallet, and Etrade disappears, nothing happens to your money. Same with a bitcoin wallet.
They are, but if you wish to exchange currency to/from real dollars, then you need to use an exchange like this, who will "trade" you BTC for currency. Unfortunately, if you had currency stored with the exchange, but did not make a transaction for real currency, it has simply disappeared.
You can also trade in person, like people have done for thousands of years with gold.
People transfer bitcoins or fiat to the service, then never withdraw.