The late J. Stowell Kenyon collated a published family history on Gill and Gell, the Gells of Kennaa in the parish of German. I do not have a personal copy, but I think there is one in the MNH library and the IOMFHS Library. I think the Marsala branch belonged to this family.
I am related through the marriage of Henry Gell to Marcia Corlett, niece of my ancestor Robert Corlett, Clerk and schoolmaster of German, being daughter of his brother Rev.Henry Corlett Vicar of German.
I am not acquainted with the descendancy of Wm Henry Gill, the composer of the Manx National Anthem, but proud to be (very distant) relative as above.
NGC