Here are my reasons for rating the movie low.
1) First and foremost, whilst I understand it is a dark / thriller murder mystery movie, you don't have to literally keep the lighting and the entire movie so dark that you can't even see what's happening on screen.
2) Sara Ali Khan has a long way to go when it comes to acting in intense roles, definitely, a great scope for improvement.
3) The plot is way too predictable, so much so, that you could simply say what's gonna happen in the next scene. Lets get over that scene of one person getting shot whenever there is a struggle with a gun. Even, the backfiring of the rifle / shotgun was so predictable.
4) Lets make better movies. Too tired of seeing similar endings.
Why is the movie named Gaslight? Is it that the movie has been shot in Gaslight and that's why it's so dark and light deprived through the movie???