The phrase "gas constant" refers to a fixed value used in scientific calculations involving gases. It helps scientists understand how gases behave under different conditions, such as pressure and temperature. Full definition
Rspecific is based on the universal gas constant at a given strength of gravitational field and it can vary according to the molecular weight of the particular mix of gases involved. (newclimatemodel.com)
At sufficiently low pressures: Pv = nrt (absolute pressure times volume equals number of moles times ideal gas constant times absolute temperature) Now a real gas equation, Van der waals: (p + a / VM2)(Vm - b) = RT.. (realclimate.org)
In that way we can allow factors other than mass, gravity and insolation to affect V without affecting T because T is determined only by the amount of KE needed to keep the mass of the atmosphere off the surface at a given height and in turn that is determined by mass (m) and the individual gas constant for the particular atmospheric composition (Rspecific). (newclimatemodel.com)