Good spotting. The MI is a huge help. This helps to differentiate the John from the other one baptized two weeks later at the same parish church (your #2, son of James G & Margaret Moore). This latter John I can't find anywhere after the 1851 census (age 13), nor the family in 1861 though father James is in Michael in 1871 with a new family.
Some additions to what is on the Ancestry website:
1. Eleanor Christian Gelling als Craine (John the joiner/builder's first wife) was buried 4 Dec 1871 at Ballaugh. She is with her 59 year old widowed mother Anne Craine at Broughjaig Moar, Ballaugh in the 1871 census as "Ellenor Gelling, Daur Mar 28 Employed at home, [born] Ballaugh" A "Catharine J Gelling, Visitor Unm 24 b Douglas" is also there. The "J" is very clear and the age is way low but I'm guessing this is Catharine Matilda Gelling who actually is 31. John Gelling is missing. Perhaps he's exploring the States for a potential move? There doesn't appear to be any children from this first marriage, which only lasted 3 years.
2. I think the second marriage to "Annie" will be Ann CORRIN md 1 Apr 1875 Braddan. They then must have gone to America because the first two children are born there, before being back to Liverpool by 1879 for child #3 and #4.
3. After John Gelling died, Annie Gelling then married Edward Feeny. The marriage is in the Dec quarter 1890 W.Derby.
4. They are in the 1891 census at 22 Tinsley St, Everton (RG12/2957 124 pg 52 #337). That page is missing on Ancestry but they can be found on FindmyPast. [I reported this to Ancestry].
5. Annie Theresa Gelling b 1879 married Edward Scott in 1898 and the grandchild Teresa Scott age 2 with her step-grandfather and grandmother Feeny in 1901 belongs to her.
6. An Annie Feeney age 49 is in the Dec 1902 deaths.
7. I'm not sure about his statement "I've found John Gelling in Liverpool boarding and working as a house carpenter in 1861". The nearest I can find is at Warrington, where a John Gelling age 24 House Carpenter born IOM is boarding with a Henry Roberts, House Carpenter (other house carpenters also boarding there). However I think this is the son of Stephen Gelling and Esther Bell, bp 21 Sep 1834 Malew. He was a house carpenter age 36 and 46 in the next two census's at Castletown.
8. I can't find John Gelling in 1861. He'd be 23 or 24. Thus, he's missing in both 1861 and 1871.