Absolutely. At hydroelectric stations, for example, the power of gravity is harnessed to produce electricity.
Your question is ambiguous. Are you asking why gravity exists or is gravity or more to the point can Gravity/Magnetism be used as an energy source.
Gravity
Generally when water is used as a source of energy it is either the kinetic energy of the flowing water or the potential energy of the water as it changed height that was used as the energy source. In both cases it is gravity that is the ultimate driving force that makes the water move (for kinetic energy) or is the source of the potential energy - which is recovered as the water moves from higher to lower.
the sun is a energy source of light.
The source of energy that is used in chemosynthesis is the chemicals.
Light energy is used . Sun is the source
The source of energy used by mitochondria is ATP.
Gravity
The source of energy in stoves is HEAT ENERGY
fule fossile is the most used energy source
Not really because the intense gravity field prevents most of the energy escaping.
Carbs! Carbs are an internal source of energy.