No one is gaslighted in this book, as no one is manipulated by causing them to question their sense of what is real, their grasp on reality, or their sanity. People are lied to and manipulated in other ways, but there is NO GASLIGHTING. Spreading misinformation is dangerous.
There is no DID depicted, but (spoiler) the villian does share someone else's body throughout the movie. Though he has his own face on the back of the head. This can be seen as a fantasy version of the "split personality" tropes (This is often referred to as "two-faced", which they applied litterly in this case) and it does include things such as the "reveal at the end", use of mirrors and other tropes associated with stigmatizing DID depictions, along with adding to fear of the idea of more than one person in one body. And that those that are more than one are the villains.