A term inspired by the US comedy series Arrested Development
. It refers to one of the show's central characters, 'GOB', an arrogant, shameless, self promoting showman.
In real world terms, "a GOB" is someone who displays the same classless showmanship, such as the lead singer of a pub covers band who thinks he's the greatest thing since Barry White but can't sing to save himself, or a standup commedian who obviously thinks his material is great, when in reality it stinks. Badly.