Answer
The gravitational PE is much smaller.
Work Step by Step
The gravitational potential energy of a molecule is given by mgh. Its average kinetic energy is $\frac{3}{2}kT$. As requested, find the ratio of potential energy to kinetic energy. $$\frac{PE}{KE}=\frac{mgh}{1.5kT}=\frac{2(15.99)(1.66\times10^{-27}kg)(9.80m/s^2)(1.00m)}{1.5(1.38\times10^{-23}J/K)(296K)}=8.49\times10^{-5}$$
This is such a small ratio that the PE may be safely neglected.
