Garthorpe is NOT one of the eight original parishes in the Isle of Axholme, but for centuries it was a township in that far north-west corner of Lincolnshire. See the histories and descriptions provided by the Isle of Axholme FHS.
Garthorpe was once part of a river island, having the Meredyke on the south, the River Trent on the east and the Old River Don on the north and west.
In the 1086 Domesday Book, the village name is rendered as Gerulftorp, from the Old Scandinavian "hamlet of (a man named) Geirulfr". [A. D. Mills, "A Dictionary of English Place-Names," Oxford University Press, 1991]