I can't claim it as a certainty, but the ages and birthplace in census match very well. I was hunting the IGI and my own database to find a Mary Gelling to match the marriage and census entries, and this was the only one that's even close. Nearest one is 1783 Marown and she's already taken care of (married Thomas Caine 1804).

For some reason I don't really understand, the Gellings hardly ever used the name "Mary" until the 19th century, when they discovered "Mary Ann" and went overboard with it
.