Hello Marg.
Yes, there's lots to think about there. I'm going to have to wait until January to really study this, as we're packing for a 2-week trip (Florida and a Caribbean cruise ... leaving tomorrow) plus organize for an early family Christmas eve tonight.
The John Gellings in the mid 1700s are a real challenge. For example, John Gelling of Camlork married a Catherine KELLY (I don't have a date) with several children in 1730-40's but it seems only one daughter Jane (married William Kneale 1773 ... contract says "only daughter of ..."). Then my ancestor's brother John Gelling (b 1718 Onchan, son of John Gelling & Mary Cotteen m 1711 Onchan) is I think the husband of Catherine KILLEY, marriage 1739 Braddan, with a son John 1740 and others. The children of these two couples are easily tangled with the similar names. I've made an attempt to sort them out, but it's dicey.
For John 1740, son of John Gelling and Catherine Killey (therefore in my family?) I have a big question mark. There was a John son of John of Douglas who died of smallpox in 1765, who I thought was this John. However, I later thought maybe this John married Judith CHRISTY, with a couple of children in the 1760's. A Judith Gelling witnessed Margaret Clark als Mylchreest's will in 1775 with a legacy to "John Gelling's wife". Margaret Mylchreest is in my family, d of Daniel Mylchreest and Mary Gelling (John's sister, b 1713).
I'll have to leave you with that for the moment. There's lots more to study and discover, that's for sure.
All the best
Eric