HCl gas is molecular HCl in the gaseous phase, HCl acid is HCl in solution with water, and can be said to be in the form H+ Cl-. (Alternatively, to be a pendant, H3O+ Cl- as technically protons does not exist on their own in solution.) It is this dissociation of the molecule into constituent ions, which gives an acid its properties.

On a physical level, HCl gas is a yellow/green gas, and HCl acid is a clear solution.