I wish I could be more helpful about Thomas G's parents. He made 6 census's, which was very nice of him. The census's are consistent for his age (40,50,60,70,80), so it works out to late 1810 to early 1811. Most said "Douglas" and one said "Braddan".
The IGI gives the following for baptisms of Thomas Gelling:
1. 11 Nov 1810 St. George, Douglas, son of John GELLING & Ellinor CRELLIN. They married in 1798 Braddan and I have 4 children: John 1803, Margaret 1805, Thomas 1810, Charlotte Elinor 1816. The first 3 were St. George, and Elinor was Marown. I haven't researched these people, so that's all I know.
2. 17 Nov 1811 Braddan, son of Paul GELLING & Elizabeth GARRETT. They married 2 Jan 1802 Braddean and I have 8 children, all Braddan. I have a note that Paul may be son of Paul Gelling & Jane Gelling of Camlork, but that might be wrong. I know little else.
3. 10 Apr 1812 St Mathews Douglas, son of Thos GELLING and Ann [MOORE]. I have him tentatively as the one marrying Anne Esther BRIDSON in 1846 at Malew with 6 children. However, I'm looking at this now and think this could be the one, and my tentative other family is wrong. Children naming pattern is good (first son is Thomas, first daughter is Mary Ann).
Continuing with the idea that #3 is the one, the parents Thomas GELLING and Ann MOORE married 15 Mar 1808 Braddan, and I have 3 children: Edward Thomas 1808, Ann 1810 and Thomas 1812. I know nothing about Edward Thomas. Ann married Henry KELLETT in 1838 Braddan and had a large family. She lived to age 93 and he lived to 82. Henry Kellett was a Schoolmaster in Douglas, born England.
Could Thomas Sr be the son and executor of John Gelling (md Margaret Caroon) 1822 will? There is no IGI baptism for him. John Gelling bp 1741 was the son of John Gelling and Ann Fayle of Ballig, and brother of Robert Gelling the Schoolteacher who we'talked before here.
However, the "Miller" trade doesn't fit here very well. That was a trade of the Ballamillaghyn Gellings. But I think these families were close, so maybe it's ok.
More questions than answers I'm afraid. Maybe somebody has some further insight?