Gammer is someone who is hideous to look at. Mainly teenage female chav types from London that used to use this phrase in the mid to late 1980s, to describe an unattractive boy.
It was believed to have originated from the pale, pink, pasty flesh of a pork gammon joint. Also termed as gammer-chops. Chops which is a slang term for the face.
What a gammer, that bloke's a right gammer. Eeeeeew yuk, what a gammer look at his face...