Gaslighting does not mean “lying.” While you are lying while you gaslight someone, the difference is that they’re supposed to make someone question something they considered to be true.
A bit late, but the I wanted to add that the distinction is something like this:
"Did you lower the lights?"
"No, I didn't"
"Are you sure? They look dimmer."
"Maybe, but I didn't touch them."
vs
"Did you lower the lights?"
"They're the same as they've always been."
"Are you sure? They look dimmer."
"They don't look any different, they've always been like this."
Like OP said the difference is in the accuser second-guessing what it was they brought up to begin with. While you're lying in both of these two cases, the accuser's perception of reality isn't ever brought into question in the former. In the latter example the accuser has to reconcile whether-or-not they misunderstood the situation, parse whether or not it was a misunderstanding on their own part, and determine if it's even worth arguing. The key in the latter is that the victim, over time, no longer trusts their own intuition and perception of events around them, and begins to second-guess and blame themselves more often than they otherwise might.
At least this has been my layman's understanding of it. Fittingly it's a narc tool, since the first case at least acknowledges and legitimizes the base concern of the accuser (did X happen?).