I know where it comes from and I have experienced the attempt.
That does not make your use of the terminology any less incorrect
I can totally see someone saying "Calm down" fitting into the definition of gaslighting. Depends on the circumstances. I'm visualizing Person A saying something insulting or upsetting to Person B. Person B subsequently gets upset or offended, whereupon Person A tells her to calm down, the implication being that there was nothing offensive about what Person A said even though Person A did intend offense.
Person A is trying to convince Person B that she completely misheard what was said, or miscontrued its intent, or has no right to be upset under the circumstances.
It's more subtle than outright telling someone she's nuts, but it's the same sort of maneuver.