Hi Gunito, thank you for joining the group and for asking one of the most important questions. Mass is simply a function of the electric charges involved: no charge, no mass, that’s why chargeless radiation like photons and “neutrinos” (should be named photinos) are massless. As for gravity, it’s simply the centripetal forces associated with pure rotational energy. An electron or positron itself spins like a whirlpool, both of them rotate around each other relativistically while also precessing as a whole — each rotational system brings about corresponding centripetal and centrifugal forces. Gravity is a result of rotation, not the cause of it, contrary to what’s printed in all the textbooks today.
Thanks again and look forward to more discussions on these fundamental questions.