Yes, the father of John Gelling who married Catharine Killey 1739 is John Gelling of the 1737 Braddan will, buried 17 Feb 1736/37. At the time the younger John (bapt 23 Sep 1722) would be 14 years old which makes him barely of age for that time. His sisters Margaret and Elizabeth were bapt 1726 and 1732 respectively, and a fourth child William bapt 1734 and buried 1736 before his grandfather's death.

It seems John was barely 17 when he married Catharine Killey who I think was up to 10 years older than him. They had over 66 years of marriage and 10 children. I don't know Catharine Killey's parentage.

Ann Gelling als Moore, relict of JG Sr., died in 1748 and left a will that mentions all 3 living children.

Elizabeth married John Cain 29 Oct 1751 Marown and as far as I know they had 8 daughters (7 lived) and no sons. John Cain died 1776 shortly after the last daughter was born and Elizabeth then married John Kissack 9 Sep 1777 Onchan but no children. She was 16 years older than her second husband. She was buried 12 Jan 1808 Onchan. Six months later John Kissack married Elizabeth's great-niece Catharine Gelling (b 1776 grand-daughter of JG & Cath Killey and eldest daughter of son William) who was 28 years younger than her husband. John Kissack died 1822 Onchan, after which Catharine married again 1825 to Charles Kewin.

I don't know what happened to Margaret, though perhaps she is the one who married Paul Fayle 31 Oct 1749 Braddan and had 5 children before she died in 1783 Marown?