The book is
"A Manx Family - The Gells and Gills of Kennaa and Malew" by John Stowell Kenyon. It was privately published and a numbered copy was sent out to all the people included.
The is probably a copy in the Manx Museum because he left all his papers there when he died a few years ago. I have one too.

I didn't realize that you were talking C18. There is a William, b Apr 1749, younger son of John Gell the 5th of Kennaa, Kirk German and Ellinor Mylchreest of Ballagarraghyn, Nr Ballacraine, but there is no record of his having been married in Kenyon's book (this doesn't mean that he wasn't - Kenyon only followed a limited number of lines. There were too many Gells!)

If this turns out to be the one that you are interested in then Kenyon has his ancestry going back to the 1st John (b 1515).

I have the data on an Excel file but unfortunately there doesnt seem to be a way of attaching to a post here.

Good hunting.