Actually, there’s a reason “gaslighted” is correct (or, at the very least, perfectly acceptable).
The reason is we are really not using a form or variant of the verb “to light” in this case. What’s happened is the name of the movie, Gaslight, itself has become the verb and it’s independent of to light. That being the case, we don’t use the past tense of to light, rather we use the past tense of this new verb, to gaslight. Then, as with most new verbs, they take the weak -ed morpheme to indicate past tense, past participles, etc., rather than the strong (irregular) forms that existed from Old English.
A somewhat similar process was brought up decades ago when talking about “Walkmans” instead of “Walkmen”, but then we’d have to get into headless nouns and such.