Everything is related to the scale that you care about.

The existence of gravity introduces a characteristic length scale h∼kT/mg, where k is the Boltzmann constant, T is temperature, m is molecular mass, and g is the magnitude of acceleration. In the earth atmosphere, this value is of O(1000). So under general laboratory conditions, the gravity effects can be omitted. But if you care about the large-scale flow motion, then you certainly need to pay attention to the gravity.

Ref: Xiao, Tianbai, et al. "An investigation of non-equilibrium heat transport in a gas system under external force field." International Journal of Heat and Mass Transfer 126 (2018): 362-379.