You're confusing force with work. Gravity is a perpetual force, but a force doesn't generate energy. To generate power you need gravity to move something downw

You're confusing force with work. Gravity is a perpetual force, but a force doesn't generate energy. To generate power you need gravity to move something downwards, releasing the potential energy. And when that thing hits the bottom of the shaft you've used up all the potential every that was stored in it.