What does it mean when people say that Gibb's free energy is the 'maximum amount of non P-V work'

In "The Principles of Chemical Equilibrium," by K.G. Denbigh, in section 2.4 discusses, in a very precise way, how this applies to (a) a closed system at constant temperature and pressure and (b) an open system operating at steady state in contact solely with a constant temperature reservoir. In the former case he discusses a specific example of non-PV work being done by an ionic reaction involving electrochemical potential.

He explains this all much better than I could ever do at paraphrasing it, particularly for the closed system case.

However, for the open system case, he shows that the maximum shaft work possible is equal to the decrease in Gibbs free energy per unit mass passing through the system, and this is much easier to understand. Moreover, the open system case does not require constant pressure, which, in my judgment, is particularly appealing.

For an open system operating at steady state (with negligible changes in kinetic and potential energy of the flow), the first law gives us $$Q-W_s=Delta H$$where $W_s$ is the shaft work, and all quantities are per unit mass of flowing stream. In addition, if there is a constant temperature reservoir at temperature T' as the only means of heat transfer, the 2nd law gives us $$Delta S=frac{Q}{T'}+sigma $$where $sigma$ is the entropy generated in the control volume per unit mass of flowing stream passing through the control volume. If we combine these two equations, we obtain: $$W_s=-Delta H+T'Delta S-T'sigma$$ If the steady flow system is operated reversibly and such that the inlet and outlet temperatures from the system T are the same as the reservoir temperature (i.e., T' = T), this equation gives as the maximum shaft work for isothermal operation, $$W_s=-Delta G$$