Marg, I' m curious why you're favouring this couple as opposed to the other two? On the surface all 3 look like candidates for William based on children's naming patterns (which we know can be flimsy evidence). Let's look at each couple in turn to see how the evidence holds:

(1) 21 Jun 1795 to John GELLING & Elizabeth OATES at St. George Douglas
- your first choice
- 3rd son named John, 1st daughter is Elizabeth Margaret, possibly named after mother and herself. (1st son was William, maybe named after himself?)
- marriage: John GELLING to Elizabeth OATES 16 Jan 1787 Braddan. 9 children on IGI from 1787 to 1810 at various Douglas & Braddan churches.

(2) 28 Feb 1794 to John GELLING & Isabel CAVENDISH at Braddan
- Braddan is the birthplace for William in all the census's where it is given (1851, 1861).
- John is 3rd son, but no Isabel amongst the 2 daughters.
- marriage: Jno. GELLING to Isabella CAVENDISH 18 Nov 1788 Braddan. 10 children on IGI from 1788 to 1810 all bapt in Braddan

(3)27 Oct 1793 to Robt GELLING & Jane KELLY at Marown.
- Robert is 2nd son but is also the name of Margaret's father
- Jane is 2nd dau
- however, Marown does not match William's census birthplace though is next parish.
- marriage: 12 [or 22] Sep 1770 Robert GELLIN to Jane KELLEY at Braddan. About 12-13 children 1771 to 1795 bapt in Marown.

I think couple #1 is on the 1841 census at Gt. Georges Street in Douglas (HO107/1465/5 6 13 pg 22)
John GELLING 70 Ind Y
Eliz GELLING 70 Y
Thos CRAINE 10 Y [maybe a grandson?]
Eliz KISSACK [?0] FS Y [can't make the age out]

1851 census at Rhenny [?] German (HO107/2523/175 pg 1 #4)
John GELLING, Head Mar 87 Annuitant b Marown
Elizabeth GELLING, Wife Mar 84 b Braddan
Catherine CUBBON, Ser U 16 House Servant b Arbory

From Brian Lawson's burial index:
GELLING John 90 Bra 23 Feb 1854.
GELLING Elizabeth 88 Bra 13 Jul 1855.
There are one or two wills that might be theirs.

Burial: GELLING William 25 Bra 3 Jul 1820 [this should eliminate one of the Williams]

I must admit that I'm liking #1 too now, but am not sure. It looks like old John was born in the mess of Johns baptized in the 1760's. I'm tangled in there too with one of mine, totally confused.