  We use TheThreeHundred project, a suite of 324 resimulated massive galaxy
clusters embedded in a broad range of environments, to investigate (i) how the
gas content of surrounding haloes correlates with phase-space position at
$z=0$, and (ii) to investigate the role that ram pressure plays in this
correlation. By stacking all 324 normalised phase-space planes containing
169287 haloes and subhaloes, we show that the halo gas content is tightly
correlated with phase-space position. At $\sim\,1.5-2\,\text{R}_{\text{200}}$
of the cluster dark matter halo, we find an extremely steep decline in the halo
gas content of infalling haloes and subhaloes irrespective of cluster mass,
possibly indicating the presence of an accretion shock. We also find that
subhaloes are particularly gas-poor, even in the cluster outskirts, which could
indicate active regions of ongoing pre-processing. By modelling the
instantaneous ram pressure experienced by each halo and subhalo at $z=0$, we
show that the ram pressure intensity is also well correlated with phase-space
position, which is again irrespective of cluster mass. In fact, we show that
regions in the phase-space plane with high differential velocity between a halo
or subhalo and its local gas environment, are almost mutually exclusive with
high halo gas content regions. This suggests a causal link between the gas
content of objects and the instantaneous ram pressure they experience, where
the dominant factor is the differential velocity.
