Thank you, Donna. That's confirmed what I thought.
I made a mistake in my question by writing:
"I'm wondering if this is William (bap 25 march 1789 Braddan) to John Gelling and Isabella Oates who married 24 Aug 1817 Braddan. They also had a son Edward bap 24 May 1818. "
It should have read:
"I'm wondering if this is William Gelling (bap 19th March 1820 Braddan to John Gelling and Isabella Oates). John Gelling (bap 25 March 1789 to John Gelling and Isabella Craine ) married Isabella Oates 24 August 1817 Braddan."
So from the MIs you supplied, Donna, it looks as though Isabella Gelling nee Oates died shortly after giving birth to William in March 1820. Then William's brother Edward died 3 years later, followed in 1825 by William's father, leaving William an orphan.
It clears up 2 mysteries. Why was William the only grandson specifically mentioned in his grandfather John Gelling's 1833 will ( he was probably brought up by his grandparents at Ballacottier Farm.)
The second mystery, which I'd never followed up on, was the identity of a 20 year old William Gelling (baker ??) in the 1841 household at Ballacottier Farm of James Gelling b 1799 (youngest brother of William's father John ) and Elizabeth nee Kelly. The 4 year old John Gelling in this household went on to marry my great grandma's sister, Esther Clague b 1843 in 1862.