First of all, thank you very much for your question! You are certainly doing some good thinking.
While perhaps intuitively this idea might sound plausible, the answer to your question is no.
While gravitational energy can be converted into other forms of energy, the only way you can get some energy out of a gravitational field is to move from an area of high gravitational potential to an area of low gravitational potential (in other words, moving from a high place to a low place). For example--when you drop a ball, its change in gravitational potential energy gets converted into kinetic energy until it hits the ground. You have to use the exact same amount of energy you got from the fall
to get the ball back to the height it started so it can fall again. You don't gain any net energy.
Is there any weasel room here? With clever thinking can we maybe come up with some contraption to work around this? No.
The reason being that this sort of thing is exactly the same (in principle) as perpetual motion machines
and clearly violates fundamental thermodynamical laws. It's important to take into account the TOTAL energy gained or lost in any system.
For instance, using the sun as an energy source works because we have a consistent influx of electromagnetic energy due to the nuclear fusion reactions which power the sun.
I hope that answers your question!
(published on 02/03/2011)