  We present for smooth non-oscillating backgrounds an analytic formula which
calculates the energy density of massive and massless particles created via
gravitational particle production, thus giving the corresponding reheating
temperature. It can be applied to models of Quintessential Inflation such as
$\alpha$-attractors, and shows that for masses larger than the Hubble rate at
the end of inflation, namely $H_{END}$, the reheating temperature is
exponentially suppressed. On the contrary, for masses of the order of $H_{END}$
one obtains a maximum reheating temperature of the order of $10^7$ GeV.
Finally, to overcome the constraints coming from the overproduction of
Gravitational Waves in Quintessential Inflation, we have shown that the viable
masses which ensure the Big Bang Nucleosynthesis success are in the range
between {$2\times 10^{10}$ GeV and $ 4\times 10^{13}$ GeV}, leading to a
maximum reheating temperature of the order {$10^5-10^7$ GeV}.
