  Reheating after inflation can proceed even if the inflaton couples to
Standard Model (SM) particles only gravitationally. However, particle
production during the transition between de-Sitter expansion and a decelerating
Universe is rather inefficient and the necessity to recover the visible
Universe leads to a non-standard cosmological evolution initially dominated by
remnants of the inflaton field. We remain agnostic to the specific dynamics of
the inflaton field and discuss a generic scenario in which its remnants behave
as a perfect fluid with a general barotropic parameter $w$. Using CMB and BBN
constraints we derive the allowed range of inflationary scales. We also show
that this scenario results in a characteristic primordial Gravitational Wave
(GW) spectrum which gives hope for observation in upcoming runs of LIGO as well
as in other planned experiments.
