There's one more child of John Gelling & Isabella Cavendish, their first-born child Mary bp 6 Jul 1788 Braddan. They married 18 Nov 1788 Braddan, so assuming these IGI's are correct, they had little Mary with them at the altar.

You seem certain that your ancestor William will be the William bp 1794 of the above couple. Was there not another possible couple, that being Robert Gelling & Jane Kelly who married in Braddan 1770, and their 13th child was a William bp in late 1793 at Marown? Have you been able to discount this one? I haven't studied any of them since the earlier research a few months ago, so have little more to add.

At this point, I have no idea who the parents of the JG who married Isabella Cavendish 1788 will be. Maybe there's a will that will tell us, but I haven't seen it yet. There are an amazing number of John Gelling's existing at that time, unfortunately.