Gaslighting the simplest explaination

 The easiest and simplest way to explain gaslighting to someone is saying

it is when someone else believes their reality to be true and make you doubt your

own reality or experiences.  This, if done over a period of time, can cause many

serious mental health conditions which could include something like the

infographic I am going to post below along with a link to where I found it.

This infographic explains simply about Emotional Abuse and the Silent Treatment is a form of this as well as gaslighting.  There is a wide range of gaslighting behaviors some of which are unintentional and somewhat harmless but the more insidious one's can cause all the above symptoms.  This is likely why gaslighting in general is not against the law but if your trying to manipulate someone or psychologically torture them (which is what the very definition of gaslighting says) then depending on how you go about doing this whether it be with technology or using a group with a false narrative of events and not allowing the person being gaslit to be heard can be very illegal and in the UK they are already passing laws to hold those who do this intentionally accountable.  In the US there has been a few cases where precedent was set on what was determined to be gaslighting, I'll provide the link to the article here.  
There is also a short Ted talk which explains what emotional abuse is and I'll provide a link here.

     I hope this clears up any misconceptions about what gaslighting is and how it can affect someone.