1) Gravity is a source of kinetic energy, but in order to use it, you must first apply energy to elevate yourself in the gravitational field. You won't be able to gain more energy than you supply, and this is why there is no such thing as a perpetual motion machine. So no, in the end you will only come out with less energy than you started.
2) As far as we know, everything with mass or energy attracts other things with mass or energy (not just mass, because light can be bent by gravity). So with our current understanding of physics it is impossible to reverse the effects of gravity without applying some other type of force.