The mouth; also a bit or morsel: whence gobbets. Gift of the gob; wide-mouthed, or one who speaks fluently, or sings well.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Goat * Gob StringNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes