  We study the thermodynamic properties -- pressure, entropy and trace anomaly
-- of a gas of glueballs that includes the glueball states obtained by various
lattice simulations. We show that this model, called Glueball Resonance Gas
(GRG) approach, describes well the thermal properties of the Yang-Mills sector
of QCD below the critical temperature $T_c$, provided that $T_c$ is properly
matched to the corresponding determination of the glueball masses, obtaining
$T_c \sim 320 \pm 20$ MeV. The inclusion into the GRG of heavier glueballs not
yet seen on the lattice, assuming that glueballs follow Regge trajectories as
quark-antiquark states do, leads only to a small correction. We consider the
contribution to the pressure of the interactions between scalar-scalar and
tensor-tensor glueballs, which turn out to be also negligible.
