Weight to electrical energy.

I am trying to calculate the output of a weight imposed on a electrical generator via a gearbox.

Can anyone please help me with this?

Let say the weight is 500 000 metric tonnes. How much electricity could this produce if putting a constant force on the gearbox?

This is a theoretical question.

Like a truck full of load rolling down a hill. Using the gearbox to brake the truck to a constant decent. How much electricity would the dynamo produce?

Best regards,

Horisize