Oh joy! Two of them! Apparently I had noticed this a long time ago, made a brief note to check, and then never saw it again. They do look like two different couples.

These are the children I've found on IGI for John Gelling & Elinor Crellin, with various spellings:

1. John Gelling bp 20 Nov 1803 St. George Douglas. He went to USA in 1830 and married Hannah Monson in 1833 in NJ - no children.

2. Margaret Gelling bp 17 Mar 1805 St. George. Possibly the granddaughter Margaret Gelling (sister to John Gelling) in Jane Gelling of Ballamillaghyn's will of 1808 (but John Gelling & Judith Quine also had identically named children at about the same time). Possibly married Paul Gelling in 1839 in London (marriage cert says father is John Gelling but no occupation provided), but this also could be JG&JQ's daughter. Could be the one who had an illegitimate daughter Margaret in 1826 Marown.

3. Robert Gelling. No IGI baptism but age 43 in 1851 at Virginia Farm Braddan works back to about 1807. Married Catharine Caine 1843 Marown, 4 children, to USA 1860 and joined brother John & sister Ellen in Illinois.

4. Catharine Gelling bp 10 Dec 1808 St. Mathews. IGI says "Jno Gelling & Easther Crellin" but presumably s/b Elinor. Nothing more known about her.

5. Thomas Gelling bp 11 Nov 1810 St. George. Was discussed Apr/May 2009 in this BB. Married Agnes Brough 1839 Lezayre, 7 children, buried 2 Apr 1893 Maughold.

6. Isabella Gelling bp 19 Sep 1813 Marown. She's a new discovery found on the IGI while researching for this note.

7. Charlotte Eleanor (called "Ellen") bp 1 Sep 1816 Marown. Joined her brother John in Sangamon Couny Illinois in 1852. Did not marry.

Given they space out quite nicely, I think they all will belong to one couple. And since they start in 1803, I'd pick the second marriage as the more likely. Perhaps the other marriage might be an older couple? Did the register say if they were bachelor/spinster?