I have John Gelling & Elizabeth (Oates) after all, on another set of notes.

KK Braddan burials
Elizth Gelling Ballagh KK German late of Ballamallaghyn Jul 13 1855 age 88
and
John Gelling Ballamilleghyn [or something like that; I can't read my own writing] died at the Ballaghe KK German bur Feb 23 1854 90 years .

So, we've done a wonderful job on this family, but it isn't Marg's of Sylvia's. The Ballamilleghyn Gellings were a well-established family going back many generations.

Having now eliminated our initial #1 candidate, we now have 2 candidates left for parents of William Gelling born ca 1794 Braddan according to the census, i.e. John Gelling & Isabel Cavendish of Braddan (Sylvia's family) and Robert Gelling & Jane Kelly of Marown. Both had a ton of kids and neither seems to have made it to the census, so this could be challenging.