Why does the Gibbs free energy not decrease ad infinitum?

I would like to ask which guarantees if the Gibbs free energy has a minimum in closed system (e.g. in a chemical reaction system) at constant pressure and temperature? Why does it not decrease ad infinitum?

• @TobiR The second law states that $\dot{S}\geq 0$, not $>0$. And it has a finite maximum because, in quantum mechanics, states are discretised, and so a finite-sized system has only a finite number of possible microstates. – lemon Jul 5 '16 at 14:04