Elizabeth Gelling is 16 in 1841 and a "granddau" unmarried 26 Dress Maker b Braddan in 1851, both at Shaws Brow Douglas with her grandmother Isabella.

I believe she matches the IGI baptism of Elizabeth GELLING 8 Apr 1825 Braddan to Felix McKEWIN & Sarah GELLING. Sarah was a daughter of John Gelling & Isabella Cavendish bp 21 Aug 1803, and she married John Shimmin 10 Feb 1833, with 6 more daughters and then finally 2 sons before she was widowed in 1850. She died 1870.

I haven't tried as of yet to trace Elizabeth past 1851, other than to note she isn't with her mother. Her father Felix McKewin (or McKeown or variations) doesn't appear anywhere in IOM but I found 4 deaths from 1838 to 1877 in the FreeBMD index that could fit, 3 of which are in Lancs/Chs.

To answer your other question re children of Wm Gelling & Alice Cavendish, James is the eldest, baptized 12 Jan 1800 Braddan. I think he may fit a burial 2 Aug 1826 age 26 Braddan.

Here's what I have for them. William Gelling married Alice Cavendish 20 Nov 1798 Braddan. I don't know either of their parentages, nor their death/burials. Note that the Alice Gelling in the March quarter 1855 Liverpool is Alice Jarrett, widow of William Gelling md 1817 Braddan, born 1787 Santon (her 1855 Braddan will is transcribed). Thus, there are two William & Alice Gellings.

Children of William Gelling & Alice Cavendish. All Braddan unless otherwise indicated:
1. James bp 12 Jan 1800, likely buried 2 Aug 1826 age 26 (no will).
2. Henry Daniel bp 19 Sep 1802 and likely the "Henry" buried 1 May 1827 age 25 (will).
3. Frances bp 22 Jul 1804, buried 20 Jul 1823 age 19.
4. Edward bp 27 Apr 1806.
5. Richard bp 6 Mar 1808. He may fit a FMP burial of Richard Gelling age 15 for 30 Apr 1823 Cardiff St John, Glamorganshire, but can't discount it being a "Gilling" without seeing it.
6. John bp 6 Mar 1808 (twin to Richard?)
7. Dinah bp 13 May 1810. She matches a marriage 22 Oct 1856 at St Johns Church Liverpool of Dinah Gelling, 45 Spinster, Brownlow Hill, father: William Gelling, Shoe Maker to Edward Isaac 33 Widower Wood Turner, Back Clare St father: George Issac, Oil Merchant. Both signed. Witn: John Porter X & Jane McEwen X. Dinah is a house servant in Lezayre in 1841 and in Douglas in 1851. Deaths W.Derby March qtr 1864 Dinah Isaac. Dinah is likely the mother of an illegitimate Alice 1842 (Laurence Kenney) and William Joseph 1845 (Richard Cowle).
8. William Joseph bp 19 Jul 1812
9 Sarah Elsey bp either 2 Mar or 2 Apr 1815 ["Elsey" might be significant?]
10. Eliza 22 Jun 1817. Possibly the burial 13 Oct 1827 age 14, but age is out.

An interesting tidbit is an entry in IOMFHS Journal May 2010 pg 54 article "Mysterious Debts": "William Gelling would not be responsible after November 19th 1838 for any Debt or Debts that his wife Alice may contract after that date.".
Which William-Alice couple is this? Upon reflection I think its the WmG-Alice Jarrett couple because they look weathier.