When I employ this term I am trying to describe a situation in which the Gnostic authors believed that they understood a hidden truth about something, and that this truth nullified the Apostolic position as ridiculous, and they found this humorous - what fools the Apostolic Christians were to believe such nonsense (or so they were trying to say)! I don't think this strategy was rhetorical. I think they were serious.
There are examples of the use of this type of strategy in other Gnostic texts. For example, Apocalypse of Peter, Second Treatise of Great Seth (which actually calls the apostolic position a "joke"), the Acts of John - all these texts laugh at the apostolic position because it is understood to be a foolish ill-informed position. Again, this does not appear to me to be rhetorical, but serious criticism that results in humor at their expense. Usually the topic centered around rewriting the passion story.
Parody is a word that literally means "beside/against a song." Hegemon of Thasos was one of the first parody writers. Aristotle refers to him. Apparently he changed some words in well-known traditional songs in order to make the songs and what they stood for appear ridiculous. Parody means "counter-song" - and that is exactly what these Gnostics were doing. They were presenting a counter-story to the apostolic one, to show it up as ridiculous. In so doing, they mocked the apostolic position. There are plenty of examples of parody in the Greco-Roman world, so it is a form that would have been familiar to the Gnostics.