I will check for monumental inscriptions when and the 1841 census when I return from some travels.
I found two other possible Margaret Gawns christened in Rushen. Daughters of:
1. Wm & Eliz Corrin, b May 2 1756
2. Wm & Margt., b Nov 14 1759
There are also christenings in Malew, Arbory, K Michael & k. German (2)

You may have found homes for some of these.
However still based on your big IF, between 1766 and 1810, there are the following four marriages in Rushen with a Margaret Gawne:

John Kelly, Nov 1, 1788, probably rules out 1772 (offspring born in Rushen 1790-1808)
John Waterson, March 2 1794 (no offspring in Rushen)
James Christian, May 5, 1802 (offspring born in Rushen 1802-1814)
Richard Sansbury, November 9, 1799 (offspring born in Rushen 1803-1814).
If I had to guess, I would plump for one of the first two as Kelly's wife, and as Richard was born in 1776, I would go with the daughter of William & Joney 1772 as his wife. Possibly James Christian and John Waterson married one of the Margarets born outside Rushen.
Bruce