according to ancestry.com :
English: from the Old Norse byname Gamall meaning �old�, which was occasionally used in North England during the Middle Ages as a personal name.
or an anglicised version of the German name Gammel.
it must be a patrinymic then, a name derived from a Christian name, like Johnson is son of John, so Gammell was originally a first name but changed to a surname to denote decendants of someone called Gammell.
Probably also related to Gemmell as in Archie Gemmell, one of my late fathers fave foottie players.