Sorry, physicists are slack in terminology, and it's not Gibbs energy.
There're many "free energies", connected by Legendre transform, and each doesn't have its name. (You can't name them all.) For magnetism, T and H form a natural set of variables, neglecting pressure. I guess Huang calls it "Gibbs energy" since its all arguments are intensive variables. But it's bad and you shouldn't follow suit.
Sometimes when you take the sum of all values of an extensive variable (say in Monte Carlo simulation) it is called "grand canonical", even if that variable is not $N$. For example in Ising model, when you fix $M$ it is sometimes called "canonical", and when you fix $H$ and let $M$ vary, it is "grand canonical".
I don't mean pressure is not important. In some experiments samples are put under extremely high pressure, but for most solids tripling the pressure from 1 atm hardly changes the material's behavior. (See also a nice comment by an anonymous user.)
BTW: When you ask next time, be sure to state exactly where it is the equation you're referring to. Not only "chap 16", but "sec 16.1" or "in eq 16.2", and also "Huang's 2nd edition". (It's not frequent, but pages can change, e.g. in epub. I don't recommend spcification by the pages.) Remember those who answer (and those who read) spare their precious time for the forum participants. (Each way of communication has its own difficulty, in person, online, etc. ;-)