Peter, I'm not sure about the legend part, but I am old so I make that qualification.

I'm still confused about the linkages. I know about everybody down from John Thomas Gelling, but who are "John Gelling and Elizabeth Gelling"?

Be that as it may, I'm not 100% that Charles G is your William's father and therefore James Mansfield G's grandfather, but the circumstantial evidence (Liverpool connection of son Wm in wills combined with perfect 1851 age and good naming of children) are stronger than with anybody else. So assuming he's who we want, here's some more detail:

Charles GELLING md Elizabeth QUILLIAM 11 May 1770 Braddan.
Braddan Burial: Charles Gelling (Hatter) Doug's aged 68 years Feb 17 1805.
Braddan Burial: Elizabeth Gelling als Quilliam Doug's aged 60 yrs Mar 24, 1805

9 children, 5 of whom were alive in Charles' 1805 admin. The 5 are:
1. John bapt 10 Feb 1771 St. Mathews Douglas. Several Johns, several potential marriages still to sort out.
2. Jane bapt 27 Aug 1775 Braddan. Not researched yet.
3. William bapt 11 Oct 1778. Was in Liverpool in 1811 when he acknowledged his rights received from his father's admin. Our choice to be the one who married Elizabeth Mansfield 8 Aug 1803 St. Anne, Richmond, Liverpool.
4. Robert bapt 24 Jun 1785 St. George Douglas. Married Martha Mylchreest 15 Sep 1805 Braddan, 8 children before she died. Then married Ellen Shimmin 19 Nov 1833 St. Johns Old Haymarket Liverpool, no children. Alive through 1861 and they perhaps match the two Gelling deaths in 1862 West Derby in FreeBMD. Martha Mylchreest's great grandparents were John Gelling & Mary Corteen of Onchan md 1711, who were my ancestors as well.
5. Ruth bapt 4 Feb 1789 St. Mathews Douglas. Married John Kneen 1 Dec 1824 Braddan, 4 children before he died 1837. On Victoria Terrace Douglas in 1851 & 1861.

The other 4 were Ann, Elizabeth, Robert#1, Ruth#1.

Charles' 1805 Admin was extracted by me from a film years ago as follows:

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1805(1) Braddan Charles Gelling 19 June 1805 of KK Braddan d 17 Feb last intestate

5 children: John, Robert, William, Jane & Ruth Gelling co-admin.

John, Rob & Jane surrendered to Ruth, Wm off the island.

Ruth and her uncle Thos Quilliam

Pledges sd John & Robert Gelling
1806 John Gelling claims from the estate of Charles Gelling £25
Letter from Liverpool 1 June 1811 from William Gelling received from hands of Robert Quilliam £2 as right from father Charles Gelling.
*******
I can tell from my writing that this will was extracted near the end of the day, so it should be checked for accuracy.

The baptism 25 Nov 1736 Braddan of Chas Gellin to Wm Gellin and Ann Crain is probably him. "Ann" should be "Catherine"?. Wills of William Gelling 1769 and Catharine Gelling 1772 [widow of Douglas, formerly of Ballavagher in Braddan] identify the children. You can see them on the Notebook. The couple married 28 Nov 1722 Braddan.

William was probably born in the 1690's or so, but there are several to choose from.