Ya hate to see it happen: a word gets so trendy, it loses its meaning. It happened with "literally" when we suddenly decided it meant "figuratively" even though it literally doesn't. Now it's happing with the term "gaslighting."
Lying is not gaslighting. It's often part of it but someone just straight up telling you untruths isn't gaslighting.
Gaslighting is when they question your perception of reality. It's not, "I didn't go out with her." It's, "you're so insecure, why would you even bring her up? Maybe you're the one who's cheating. You get upset every single time I leave. You need therapy. Something's up with you."