Gob is a slang word for mouth but it is also a legitimate English word meaning a lump of something; a mass, as of something soft; informally a large quantity or amount; waste material produced in coal mining, consisting of clay, shale, etc.
Back to slang I need to mention here that to ‘gob on’ something means to spit on it (usually after the delightful habit of sniffing up mucus first).
But according to Emma Harding in Nick Parker’s ‘Bling, Blogs and Bluetooth’ (2006) a ‘gob on’ has recently become a valid English expression as well. It means “any period detail on a house which obstinately refuses to belong to the era of the building to which it is attached”. Many such appendages are so obscene that one might be tempted to gob on them.
Aren't you glad you just learned all that?