Thomas Gelling's father was John Gelling bp 4 Oct 1741 s/o John Gelling & Ann Fayle.
His mother was Margaret Caroon bp 20 Feb 1742/43 Braddan d/o John Caroon & Catharine Brew.

John Gelling md Ann Fayle abt 1732 likely at Marown (marriages before 1733 missing). 8 children from 1734 to 1753: Ann, Margaret (d young), Catharine, John, William (d young), Patrick, Jane (d young), & Robert. John was of Ballig Marown when he died, buried 1 Feb 1784 Marown. Ann was buried 31 Dec 1791 Marown. She left a will but I don't know anything more about her.

From D Douglas Manx BB Jul 12/08
"#309 St Runius Marown
Here repose the remains of JOHN GELLING of Ballig who departed this life January 30th 1784 aged 79 years
also ROBERT GELLING who was Schoolmaster of this parish forty years he died on the 28th of January 1815 aged 62 years
also of THOMAS GELLING Mason son of the above sd ROBERT who died the 2nd of July 1849 aged 68 years
also MARGARET wife of the above THOMAS GELLING who died July 28th 1856 aged 72 year"

This John Gelling 1705-1784 was bapt 18 Nov 1705 Braddan, the illegitimate son of John Gelling of Ballamillaghyn and Ann Kneale. He was an acknowledged son and was the executor of his father's will in 1733. There is a huge number of descendants. Much has been written on this BB about the Ballamillaghyn Gellings.

Ann Kneale was buried 27 Feb 1713/14 Braddan, Her will mentions an unnamed son with legacies in the hands of Jn Gelling. The son John would be age 8 then. Also mentions her brothers and sisters including a couple of Carrooins.

John Gelling of Ballamillaghyn was bapt 18 Feb 1669/70 Braddan and buried 25 Feb 1732/33 Braddan. He married Susanna Moore d/o Capt John Moore of Santon abt 1687. His legitimate son who inherited Ballamillaghyn was also named John (1690-1737). There was a daughter Jane 1694-1742 wife of Patrick Kelly md 1713 and an earlier son William who likely died young. Susanna died 1731 and her will mentions "John Gelling a illegitmate son of her husband a heifer that was due to him by his mothers goods"

I don't know much about the Caroons. Catharine Caroon als Brew was buried 21 Feb 1773 and left a will that mentions her grandaughter Margt Gelling, dau Margt, dau Joney Caine w/o Wm. The g-dau Margt died as a baby. John Caroon is mentioned as a grandfather to the children of his daughter Margaret's 1781 admin.
A John Caroon was buried 20 Mar 1783 Braddan but I don't know if that's him or not.

The earliest known Gelling ancestors are John's parents William Gelling & Joney Christian of Ballamillaghyn. They would have been born ca 1630 and married ca 1650. He died 1683, she died 1688, both left wills. They had 7 daughters before a first son John 1667 who died, then a final child the second John 1670 who inherited when he was 13 and married at 16 or 17 shortly before his mother died. .

There is an earlier will of a William Gelling 1664 who left his share of "Baldell milleat" [Baldwin Mill] to his nephew, his [unnamed] brother's son, William Gelling. This nephew likely is the above William who died 1683. Based on children's naming, I'd guess the brother's name would be John.

Eric