I get that Gordon's Granny mentioned Megicula and having some form of connection regarding a ancestor's pact and curse. But I don't recall her exactly saying they summoned the devil.
While that does seem to make sense, the fact the words are still kinda broken up (the pauses in between and mumbles) and how this family is filled with "communication problems and misunderstandings" (like with Gordon's father), it could be something else. Those pauses (the "....") and mumbles could've been vital infromation that could easily lead into something else on the identity of their pact/contract and its connection to Megicula's curse. Not to mention, the "process" also involves forbidden magic and we've yet to know if this pact/contract is even magical at all. Maybe it's not something magic but simply a family promise or vow for somehow getting invovled with Megicula like Noelle's mother. The point being, it still seems to be up for "interpretation" until they actually say/reveal it.