From the info you have shared here, it does not necessarily sound like gaslighting and could just be that she put down the wrong day in her calendar in the first place by mistake and got confused. But if she has these tendencies in general, then it's definitely important to bring it up, I think.

My first T has issues similar to your concern. He is pretty bad at administration and remembering/recording practical things correctly but does not seem to be doing much to help it. We had a standing appointment same day and time each week so that wasn't a problem, but canceling was. He claimed not to remember my advance cancellations and called me on the phone three times, asking where I was. When I complained, he said that I should remind him the day before. The last time this happened, he kept saying I did not cancel the session. Also, he messed up the insurance claim form several times in a few ways. He typically did not apologize for these things even when he admitted it was his mistake, and were often defensive. Then interpreted my anger about his messing up as displaced anger that I had for my mother. He was generally very open to discussing my criticisms but did these things in the moment many times without acknowledging or apologizing. Of course I was annoyed, and not at my mother. I don't think that the situation has deeper meaning for me in this case, I was just reacting to his messiness.