  Inflationary reheating via resonant production of non-minimally coupled
scalar particles with only gravitational coupling is shown to be extremely
strong, exhibiting a negative coupling instability for $\xi < 0$ and a wide
resonance decay for $\xi \gg 1$. Since non-minimal fields are generic after
renormalisation in curved spacetime, this offers a new paradigm in reheating -
one which naturally allows for efficient production of the massive bosons
needed for GUT baryogenesis. We also show that both vector and tensor fields
are produced resonantly during reheating, extending the previously known
correspondences between bosonic fields of different spins during preheating.
