GoxUSD can be redeemed in places other than Gox, and is indeed considered currency in OTC trades for PPUSD, WU, BTC, etc.
Did you have 2FA enabled? If so, it's likely an indication this might not be isolated. Maybe you used a service like LastPass?
No I don't use services like LastPass. And unfortunately I did not had 2FA enabled. I was not using my MtGox account until very recently when I began selling my Bitcoins.