Moore ( http://www.isle-of-man.com/manxnotebook/manxnb/v08p153.htm) has GILL and GELL, contracted from Giolla, 'a young man,' (see MYLCHREEST, KILLEY,) are almost certainly the same name originally, the former being the earlier form. They are now used indiscriminately.

GELLING, contracted from Giolla Eoin, 'John's servant.'
thus both share a common root noun