  We investigate the stochastic gravitational wave (GW) spectrum resulting from
graviton bremsstrahlung during inflationary reheating. We focus on an inflaton
$\phi$ oscillating around a generic monomial potential $V(\phi) \propto
\phi^n$, considering two different reheating scenarios: $i)$ inflaton decay and
$ii)$ inflaton annihilation. We show that in the case of a quadratic potential,
the scattering of the inflatons can give rise to larger GW amplitude than the
decay channel. On the other hand, the GW spectrum exhibits distinct features
and redshifts in each scenario, which makes it possible to distinguish them in
the event of a discovery. Specifically, in the case of annihilation, the GW
frequency can be shifted to values higher than those of decay, whereas the GW
amplitude generated by annihilation turns out to be smaller than that in the
decay case for $n \geq 4$, due to the different scaling of radiation during
reheating. We also show that the differences in the GW spectrum become more
prominent with increasing $n$. Finally, we highlight the potential of future
high-frequency GW detectors to distinguish between the different reheating
scenarios.
