Several Gravel researchers have looked for the marriage of Pierre Gravel and Marie-Cecile Cloutier and have not found it anywhere in Quebec.I've extracted Gravel baptisms, marriages and burials from the parish records of most of the parishes on the north shore from Montreal to Chateau Richer and have not found the marriage. And, as you point out, it is not in any of the standard secondary sources for Quebec. Further, I have not found the baptismal record of Marie-Cecile Gravel.
My best guess is that the priest had a bad day and simply recorded the wrong name in the marriage record for Marie-Cecile Gravel's mother, or for both her mother and father. Or, perhaps he recorded the wrong surname, and it is something other than Gravel.