Answer by Anita Sanz, Clinical psychologist:
Gaslighting is so disturbing mainly because it causes a person eventually to do these two things, both of which are incredibly damaging.
First, gaslighting causes you to distrust yourself and your ability to perceive reality correctly. By getting feedback from the gaslighter that "you didn't see what you thought you saw, you didn't hear what you thought you heard, you aren't experiencing what you think you are experiencing, and you can't remember things correctly," you begin to believe that there must be something very wrong with you.
Being gaslighted over time effectively disconnects you from yourself, your feelings, your ability to know what you want and don't want, what you know to be true about yourself, others, and the world. It can eventually strip a person of their core sense of themselves, leaving them feeling dependent upon the gaslighter to define reality and provide approval and confirmation of what is real.
You begin to distrust your ability to know whether something is true or not, whether you really did something or not, whether you even really felt something or had a particular intention or not.
Instead of inhabiting yourself, owning your thought process and having a strong sense of self, you become untethered, uncertain, unsure of what you're doing, feeling, and who you are.
Maybe the alternate version of yourself or of reality offered by the gaslighter is true, you begin to think. Maybe you've been deluded about yourself and the world your whole life. And if that's true, then nothing you thought you knew is real. You are not who you thought you were. You have to start over, except you can't even trust yourself to figure it out. You are weakened, you are disempowered, and you are lost.
Second, once you realize what has been happening, that you have been being gaslighted, lied to, or manipulated in such a way that another person has been trying to convince you that reality is not reality, you will not trust another person completely for a very long time, perhaps never again.
When you realize that it is possible for someone to think it is actually okay to go to any length to convince you that you are not feeling, thinking, intending, or experiencing what you think or say you are, you will have a difficult time ever believing that someone you let get close to you won't also do this to you. Just as it leads to major damage to the ability to trust one's self, it leads to major trust issues in relationships.