General relativity already includes source terms in it (the stress-energy tensor, including energy and momentum densities) just as Maxwell's equations for electromagnetism have source terms (charge and current densities). So at that level the two theories are operating the same way.
There's now a quantum version of the E-M theory but no established quantum version of the gravitational theory, so in that regard they're different. Perhaps by "another theory" you mean the quantum version, in which case we hope the answer will be yes.
Mike W.
(published on 01/06/2013)
