  We provide a detailed study of gravitational reheating in quintessential
inflation generalizing previous analyses only available for the standard case
when inflation is followed by an era dominated by the energy density of
radiation. Quintessential inflation assumes a common origin for inflation and
the dark energy of the Universe. In this scenario reheating can occur through
gravitational particle production during the inflation-kination transition. We
calculate numerically the amount of the radiation energy density, and determine
the temperature $T_*$ at which radiation starts dominating over kination. The
value of $T_*$ is controlled by the Hubble parameter $H_0$ during inflation and
the transition time $\Delta t$, scaling as $H_0^2 [\ln(1/H_0\Delta t)]^{3/4}$
for $H_0 \Delta t \ll1$ and $H_0^2 (H_0 \Delta t)^{-c}$ for $H_0\Delta t \gg
1$. The model-dependent parameter $c$ is found to be around 0.5 in two
different parametrizations for the transition between inflation and kination.
