Einstein proposed a quadrapole model for gravity waves - but why? If we consider two particles of finite mass like an electron and positron that combine to convert all their mass energy to a pair of photons - - the distortion of space due the conversion of the gravitational mass to radiation should proceed at the velocity c in every direction - this implies a compressional wave. In other words, the change in the local curvature of space as measured by the radius excess [G/3c^2]M (where M is the combined mass of the positron and electron) is going to spherically
propagate (diverge) at velocity c so that a gravitational detector at a distance d (e.g., another mass M2 that is pulled toward the position of the electron and positron immediately prior to their combining) will experience a gravitational force decrease at the time t = d/c. But this force change is along the line of action connecting the center of the mass M2 with M - which implies a compressional wave.