What a tangle of intrigue this all is. I've been focusing on the Gelling side and still think it's Thomas Gelling bapt 2 Apr 1833 Malew s/o William Gelling & Elizabeth Callow. Here's why.
In 1851 he's at Ballavodden Malew farm as a labourer aged 18. On 5 Mar 1854 "Thomas Galling" signed up for service with the RN, born "Malew Isle of Man" date of birth "30 March 1833". Thomas Galling commenced service for 7 years on the "Boscawen". Private "Thomas Golling" got a medal for service in the Crimea and was a POW 21 Dec 1854. In the 1861 census "Thomas Galling" was on the "Miranda" Vessels at Sea with the Royal Navy, single aged 28 born at "Mallew I of Man".
On the RN Registers of Seamen's Services, for Thomas Galling it says his first service date: 1 Sep 1861 (which is slightly after census date), first ship served on: "Miranda" engaged for 10 years.
His discharge statement says he completed his 10 years 1 Sep 1871 and was assigned to a couple of other ships until he was pensioned 15 April 1874. His character was VG (Very Good), he was 5' 11", black hair, blue eyes, fair complexion, no wounds.
Next time we hear of him: "Thomas Galling" (written in round careful letters) witnessed his half-brother John Bridson's marriage October 1880 Malew. In 1881 census at Crossack Malew he is "Thomas Gelling", Half Brother Married 45 Pensioner b Malew, with John Bridson, his new wife AnnJ, and Thomas Gelling & John Bridson's mother Elizabeth Bridson widow age 80.
Noteworthy I think is that John Bridson's occupation at his marriage was "Overseer of Uniber Works" and this same occupation "Overseer of Uniber Works" is in the 1881 census for him with the words "dye paint" written under it. I've never heard of "Uniber" before and google didn't help, but it must have something to do with Dye Paint.
During the early to mid 1880s the family of Matilda Killey show up and now they're using Gelling or Gellen as their name. The daughter Mary Alice's 1886 marriage says her father "Thomas Gellin" was a Dyer. This happens to match the occupation of her "father Thomas Killey" in her birth certificate of 1864 in Ovenden Yorks.
Thomas Gelling married his first cousin Jane Callow 24 Dec 1885 St Thomas Douglas. He was 52 Widower Naval Pensioner father William Gelling deceased labourer, and she was 50 Widow Boarding House Keeper father Paul Callow deceased Gardener. His signature was "Thomas Gelling" and doesn't look anything like the 1880 signature of Thomas Galling. Jane's father Paul Callow was younger brother of Thomas Gelling's mother. As an aside, but true to this family's story, Jane Callow was illegitimate and her father actually was James Cottier bapt 25 May 1834 and mother was Elizabeth Hadwin (surname seems to have variations). 1841 census for the family says Jane Cottier age 7 with the younger ones being Callow. Paul Callow married Elizabeth 1838 at Braddan. Thomas's wife Jane died 1894 and Thomas married again 1899 to a Mary Ann Cowley some 23 years younger than him, but the marriage certificate isn't on Family Search. He died 1908.
So my theory is that Thomas Gelling didn't want it known he was spending shore time with Matilda Killey, so she changed the information in each of the children's certificates so he was recorded as Thomas Killey (or missing entirely in the 4th child). The maiden name of Gornal is a total mystery and I wonder if it's either a fabrication or somebody she was "adopted" by.
While in the Isle of Man in the 1880s, Thomas Gelling learned how to spell his name properly and the Gelling, as opposed to Galling, appeared. Maybe some Yorkshire accents got into the act too.
Lastly, who was Thomas Gelling married to if it wasn't Matilda. Thomas married Jane Callow before Matilda died, so if he was married to her it was bigamy and she was there on the Island to prove it. I think he was married to somebody else.
My other theory I can't prove is that the marriage 4 Nov 1865 at Aghada Cork Ireland of Thomas Gelling single age 30 father William Gelling to Sarah Sinnott single age 22 father James Sinnott might be him. But I can't find anything more about them.
Lots of food for thought here.