So let's consider a universe in which there is only a positronium pair P and a single, neutral lepton L. Clearly the gravitational field of P will attract L which accelerates towards P (it works for apples, and we assume so for all particles). Then P disintegrates into two photons which, in my simple universe, fly off at right angles to the track of L. Does L continue to accelerate towards where P was?
Those photons will still have a gravitational field. Rest mass isn't needed for that. Energy will do it too.