  We consider the direct $s$-channel gravitational production of dark matter
during the reheating process. Independent of the identity of the dark matter
candidate or its non-gravitational interactions, the gravitational process is
always present and provides a minimal production mechanism. During reheating, a
thermal bath is quickly generated with a maximum temperature $T_{\rm max}$, and
the temperature decreases as the inflaton continues to decay until the energy
densities of radiation and inflaton oscillations are equal, at $T_{\rm RH}$.
During these oscillations, $s$-channel gravitational production of dark matter
occurs. We show that the abundance of dark matter (fermionic or scalar) depends
primarily on the combination $T_{\rm max}^4/T_{\rm RH} M_P^3$. We find that a
sufficient density of dark matter can be produced over a wide range of dark
matter masses: from a GeV to a ZeV.
