Trouble with potential energy

Hi everybody, I'm reviewing for the MCAT (it's been a while), and I'm having a lot of trouble understanding potential energy.

In my review book (and a whole bunch of other googled sources), it defines energy as the capability to do work.
It also says work is something along the lines of, the transfer of energy.

On Wikipedia, I read that potential energy is the energy associated with an object due to its position.
So...putting it all together, potential energy is the capability of an object to do work due to its position.

I understand that the potential energy got there by doing work on the object, bringing it to its present position.
From what I understand, this potential energy is transferred to kinetic energy due to the gravitational force.

What I don't understand is.... The definition says the potential energy is the object's ability to do work How is it doing work?

I thought the work (conversion of potential energy to kinetic energy) was due the gravitational force acting on the object.

Did I totally miss the point of all of this?

P.S. I forgot to mention (although I think it's similar for other types), I'm asking about gravitational potential energy.

Thank you in advanced.

Just a quick edit: I was wondering why the definition couldn't be something like:
Gravitational potential energy is the object's capability to have work done on it by gravity?