Yes, gaslighting - here a complete coward's tactic to try and get a bit of very unsavoury news past you with minimal impact.
'Oh! But I told you about it at the time (translation: you've had your chance to say your piece about this so you can't get all angry with me now')
Laughable that he thought he'd get that past you - hmm, thick as well as a cheating liar, what a catch - laughable that it would be AT ALL possible that you would just 'forget'.
Now, I think we can say with certainty that, given the lengths he's gone to to lie and lie and lie about this one, and try and gaslight his way out of being held to account for either the pregnancy OR carrying on lying when you're supposed to be trying to work things out - that if what OW says is true and it could never have been his there's no way he'd have breathed a word.
So she's lying about that one - fine, what else can you expect - no problem there :)
Facts appear to be that far from trying honestly to rebuild things and be honest with you and RESPECT your right to know the score if you're going to re-commit to him and the relationship - he's continued lying all the way through, continued being a cheating coward whose main concern is what he can get away with. So he hasn't changed, he's still a scumbag, and is quite categorically not worth the shit on your shoe.
So I would give myself the satisfaction of comprehensively dumping him asap - clothes in bin bags, OUT.
You deserve a lot better :)