The most typical way of understanding coercion is in terms of a proposal - by the person doing the coercion - to make the other person's situation worse off, relative to some reasonable baseline, unless the coerced person takes some action that they otherwise wouldn't take.Coercion though is tricky to define. It's tricky in law and tricky in gaming. If the GM says "if you go that way, I have nothing prepped and we'll have to end now and play next week" -- is that coercive? Sort of feels like it to me, but it's not forcing the action, just making it tricky, so I'd probably say not coercive. But I wouldn't argue if you thought otherwise.
A GM who says "I've got nothing prepped, so we'll have to stop" is making the players worse off relative to the baseline they get to play what it was that they wanted to play. But I don't think that's a reasonable baseline to adopt for identifying coercion, as the GM is not under any obligation to ensure that that baseline is satisfied. I mean, it would be great if they did - but they're just another ordinary person participating voluntarily in a leisure activity.
(Paid GMing is different. I'd expect the contract to have something to say about this, which would then help establish a reasonable baseline against which coercion can be ascertained.)