The cheeks. To look rosy about the gills; to have a fresh complexion. To look merry about the gills: to appear cheerful.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Gill * Gilly GaupusNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes