The term has expanded in its use from the original idea of one person making another person doubt their sanity.
In current usage, it refers to any relationship abuse that seeks to distort reality to include things as far as, I think, major deceptions or major lying which puts a partner in another reality. Of course, in any given situation, either one or both people might actually be crazy or half-crazy but that is not about gaslighting.
Regardless of the extent of the sadism, narcissism, or destructiveness involved, it's a real thing and it is evil: Gaslighting.