Ironically your hypothetical UMLFs (ultra-massive life forms) would need EM-based sensory apparatus to detect gravitational waves, by responding to changes in their relative dimensions as the ripples pass through. That's the basis of LIGO and it's difficult to imagine a non-EM-based alternative because a gravity-based sensor would distort as the wave passes through and thus be "blind" to its influence just as a particle or object moving through curved spacetime is unaware that it's traveling along a geodesic.