Why does a positive output gap imply workers working overtime?
A fisherman usually works 50 hours and catches 50 kg of fish every week. So let's say that his potential (or &quot;natural&quot;) output is 50 kg of fish.
Now suppose his boss orders him to work 80 hours per week and catch 80 kg of fish. This 80 kg of fish is his real output and exceeds his potential output of 50 kg. This is not necessarily a good thing and is probably not sustainable:

He may prefer to work less (have more leisure rather than the consumption goods that his additional work can buy).
He and his fishing rod may be overworked and thus wear out more quickly.


A definition of potential GDP:

Potential gross domestic product (GDP) is a theoretical concept that means different things to different people. To some, it reflects a world in which every worker is matched with the perfect job, every good idea is implemented, and the bad ones are ignored. In this world, resources are allocated optimally with no distortions from the tax code, information frictions, or suboptimal government policies.†

I would add also that potential GDP at any moment is that which can be sustained over the long term, holding constant resources and technology at that given moment.

†This idealized definition isn't actually used to get estimates of potential GDP. Instead, such estimates are simply obtained by smoothing out GDP fluctuations.
If this idealized definition were actually used, then it would in practice be quite impossible for real/actual GDP to ever exceed potential GDP.