It's about making someone question the validity of their perception of reality. It's emotional abuse, not simply tricking or lying to someone.

When I was a kid, my parents weren't gaslighting me when they convinced me the tooth fairy was real by putting money under my pillow and taking the baby teeth. They weren't making me feel like I couldn't trust my perception of reality, or that my feelings were invalid.

(Real world example): My best friend as a teenager tried convincing me he wasn't trying to seduce my girlfriend at the time. He convinced me that my expressions of discomfort with all the "accidental" touching was me being a prude, and when I told him I thought he had ulterior motives trying to hang out with her alone and swim in his pool so often he convinced me that I was being up-tight. Lo and behold, one day in a drug-fueled stupor he admitted to me that he loved her the whole time. Making me feel like I couldn't trust my own feelings on the matter was gaslighting. Now I have trust issues.