The pattern underneath
Repeated contradiction of witnessed events, isolation from third-party memory, and punishment for trusting your own notes. The target's anxiety then 'proves' they are unstable. That loop is the harm.
How the phrase gets misused
Disagreement, forgetfulness, and a bad meeting are not gaslighting. Inflating the word makes it harder to name the actual crime.
A more precise frame
Ask: is there a campaign against the person's reality, or a conflict about interpretation? Only the first deserves this word.