Not all manipulation is gas-lighting. Gas-lighting is when you intentionally create a scenario to make another person doubt their competence as a distraction from your own nefarious actions.
If the boss had intentionally given the subordinate a task that was harder than it seemed in order to make the subordinate feel inferior, causing the subordinate to feel at fault for the lateness of a larger project and therefore not report the boss' bad decisions up the chain of command, that would be gas-lighting. But just telling them they are at fault is not gas-lighting.
To clarify, I don't mean to say the boss did something ok, just that I think the term gas-lighting as a term is very useful to describe a particular phenomenon in various relationships, but is less useful if it gets broadened. I think this is something different, not necessarily better or worse.
If the boss had intentionally given the subordinate a task that was harder than it seemed in order to make the subordinate feel inferior, causing the subordinate to feel at fault for the lateness of a larger project and therefore not report the boss' bad decisions up the chain of command, that would be gas-lighting. But just telling them they are at fault is not gas-lighting.