A lot of gas-lighters rely on the fact that you are too shocked or confused to argue with them in the moment. If you have someone who regularly gaslights, prepare a few answers for next time. Stand up to them and insist on the facts rather than their version, (but only if you’re physically safe to do so).
“I know what you’re doing and it doesn’t change the facts”.
“Don’t try to make me doubt what I know is the truth. It’s not working.”
“I’m not fooled by your attempts to gaslight me.”