In the context of hybrid inflation at the scale of grand unification, it is shown that, if (some of) the gauge bosons of the unbroken grand unified group obtain a flat superhorizon spectrum of perturbations from inflation they can affect or even generate the observed curvature perturbation and simultaneously produce a sizable primordial magnetic field capable of explaining galactic magnetism. The mechanism employs the vector curvaton idea for a supermassive grand unified gauge boson after the breaking of grand unification.