Classic gaslighting. Claim what you think is true and then ignore all the evidence that proves you’re wrong.
Point of order: I don't think this is 'gaslighting'. It's 'talking sh!t', 'being gleefully ignorant' and, possibly, 'trolling'. But gaslighting is about manipulation, and he's not trying to maniuplate anyone. He's not trying to distabalise our own knowledge and memories.
Trump lying about inaugaration crowds wans't gaslighting either. It was lying. Literally nothing about it was designed to make people who knew it was a lie doubt that.