No, I'm not descended from these Gellings, I'm descended from a Corjeage/Cavendish line.

There are two Isabellas born at an appropriate time to be married in 1788, that I know of anyway:

Isabella d/o William Corjeage & Isabella Moore c.29 Dec 1766 St Peter Liverpool (SS)
Isab: d/o John Corjeage & Christ: Cannon Oct 4 1767 KM reg

and two marriages:

John Gelling & Isabella Cavendish otp 18 Nov 1788 Braddan ipo Tho. Curphey, William Kelly
Charles Cain & Isabel Corjeage botp married 23rd June 1792 KM ipo John Kaighin farmer

Isabella Cain alias Cavendish died Nov 3 1848 aged 83. KM F4 [Charles Cain died Oct 1st 1829 aged 66. KM F4]

I can't see at the moment how I came to decide which Isabella married which husband. Since I read your message I've been looking for maybe a Cain will that might have clinched it, but I'm going to have to go to work without finding my evidence, which I hope I had in the first place!

For interest, my line is:
William Corjeage father of Isabella was a son of John Corjeage 1694 & Christian Bodaugh who also had a son Richard Corjeage 1739, father of
Thomas Corjeage/Cavendish 1785 f/o
Richard Cavendish 1824 f/o
Thomas Richard Cavendish 1846 f/o
Thomas Richard Cavendish 1878, my grandfather.

I need to get a look at the MI for Isabella to check the age, just to put my mind at rest.