Why does the Gibbs free energy not decrease ad infinitum?

Interatomic potentials have a finite minimum which means that the overall enthalpy of the system has a finite minimum. And any finite system also has a finite maximum entropy.

As this is a closed system but not an isolated system, so we can add heat to the system. And, at the same time, we can increase its volume. By doing so, we may be able to keep temperature and pressure and increase entropy (note microstate increases because the position opportunity is increased). If this can reduce Gibbs free energy continuously? I guess there is a limit because the particle speed is capped by the light speed. At certain volume, adding heat may not be able to keep up pressure. By then, the volume can not be further increased as well. So Gibbs free energy cannot decrease forever.