Hi Marg,
As you know, I had been nervous about claiming the parents of WmG b 1794 as being John Gelling & Isabella Cavendish without another piece of evidence. I think the piece of evidence is Robert G (1792-ca1840), who fits to this couple very well (and not to any others), and whose wife's family (with Robt's son Edward age 5) were living at the next farm to William Gelling in 1841. So I think I'm comfortable enough now to put them all together.

By the way, your note says "Elizabeth Cavendish" but I think you meant "Isabella Cavendish".

Regarding the two will entries for Robert Gelling 1841, Kay Clegg has the film and tried to make them out on a very poor film. The first one in Book 1 is a petition of "Margaret Gelling of the town of Douglas widow". Notice to "Jno Gelling mason". "Robert Gelling your petitioner's husband sometime ago departed this life intestate". "That it is necessary that petitioner or some other fit and proper person should be appointed administrator of the Estate and Effects of the said Robert Gelling." "Granted 11 May 1841." The rest of it was too light to read.
Kay also said that the second one in Book 2 looked to be the same person, and had pledges Robert Gelling of Douglas and Thos Qualtrough of Douglas, both of whom signed as opposed to marks. Administration granted 2 Feby 1841.

The Book 2 date is before the Book 1 date, so perhaps one of the dates isn't right. She said the entire film was awful.

I don't know who "Jno Gelling mason" is, I wonder if it could be their eldest son born 1818? I haven't anything on him. He was born in 1818 so is old enough to administer. The widow is petitioning to him, so there must be a connection of some kind and a reason for the petition. The deceased also had an older brother named John b 1790, eldest son of John G & Isabella Cavendish, who I also know nothing about.

To answer your question on the two children named Robert of RG & Margaret Crellin:
I have the following Braddan burial:
Robt son of Robt Gelling & Margt Crellin aged 2 yrs 6 mos Oct 2 1823.
The IGI has a patron submission for him bp 1 Apr 1821 Douglas.
The second Robert will be the one bp 25 Nov 1827 St. George Douglas. He is 13 in the 1841 census with his mother at Athol Court, Douglas, but I can't find him after that.

The IGI has another Robert bp 26 Jun 1825 St. George Douglas. However there is a Jane in 1841 age 15 with her mother, who later married Nathaniel Brew in 1849 and emigrated to San Francisco. She is 54 in the 1880 US census. Jane isn't in the IGI at all. I tend to think the 1825 Robert should have been Jane.

Anything else you need?