Recall Monatomic ideal gas derivation and Phase space and Microstates and Macrostates
If we consider gravity, then we need to consider the position and momentum together (this is because in the general relativitiy, the gravitational field (metric) is dynamical and sourced by the full stress-energy tensor, which includes momentum => o the geometry depends on both position and momentum, making the separation impossible).
If there is no gravity, then we can split the phase space into configuration space and momentum space.

• The density of configuration microstates depends just on the volume: $\mathrm{\Omega }(x,y,z)\propto V$.
• Total number of microstates is the product of configuration and momentum microstates: $\mathrm{\Omega }=\mathrm{\Omega }(x,y,z)\times \mathrm{\Omega }(p_x,p_y,p_z)$.
• Since entropy is defined in terms of the log, it is the sum of configuration and momentum entropies: $S=S(x,y,z)+S(p_x,p_y,p_z)$.