"gaslighting" has a steady upward curve the last 10 years.
My personal guess is that it has to do with psychology and mental health awareness going mainstream due to social media and psychology channels on YouTube.
https://trends.google.com/trends/explore?date=all&geo=US&q=g...
My best guess is that it has come up as part of the Amber Heard / Johnny Depp trial and that has spurred a lot of people searching for it.
[1] Past 5 years, March & earlier - https://trends.google.com/trends/explore?date=2017-03-01%202...
[2] Past 90 days including giant spike - https://trends.google.com/trends/explore?date=today%203-m&ge... (if you go to past year you can see the spike)