Oh, Eric, my head is pounding!! Gellings are VERY confusing.
I'm gathering the bits and pieces in my database but have no hope that I will live long enough to sort out even the Johns.
There are NO Onchan MIs that I know of for any of these early Gellings--there are only burial records.
I suppose the wills are going to be crucial in separating them out. As Frances pointed out, there are some great will summaries on ManxNotebook.

The Catherine Gelling 1770 Braddan as you have said is Catherine Christian Gelling 2nd wife of Charles.

However, there is Onchan 1784 A 17 GELLING John d 10 jan 1784;of douglas;son robt;wife unnmd alive;

This looks like your John b 1718 and if so, wife Catherine is still alive in 1784