Have you heard the joke about the gas lighter?

The term “gaslighting” refers to a form of psychological manipulation where someone tries to make another person doubt their own memory, perception, or sanity. The joke you’ve mentioned plays on this term. When the person says, “Yes you have,” after asking if you’ve heard the joke about the gas lighter, they’re asserting a false statement as if it were true, which is a form of gaslighting. It’s a meta-joke that uses the concept of gaslighting both in the context of the joke and as the mechanism of the joke itself. The humor arises from the unexpected twist on the word “gaslighter,” which one might initially think refers to a device for igniting gas, but in the punchline is revealed to refer to the act of gaslighting.