So there's three categories we need to understand before classifying G'huun: Old Gods, Wild Gods, and Loa. What are they and what are the differences between these entities?
Old Gods are entities that originate from the Void. They were created by powerful beings from another reality/universe called void lords to aid them in entering our (re: the player character's) reality. Azeroth used to be ruled by four powerful Old Gods: C'thun, Yogg-Sargon, N'zoth, and Y'shaarj. Of these four, Y'shaarj is dead and N'zoth has not yet been encountered (although there are substantial rumors and foreshadowing that it will make an appearance in this expansion).
Wild Gods are powerful beings born of life and nature. Some such as Cenarius, Goldrinn, Ursoc, etc. are called the Ancients and are bound to the Emerald Dream. When their physical manifestations die, they return to the Dream to be reborn and can eventually return to the physical world. The August Celestials from Pandaria are also Wild Gods, although it is unknown whether they have any interaction with the Emerald Dream.
Loa are beings that are the objects of troll worship. This makes them unlike Old Gods and Wild Gods in the sense that what makes an Old/Wild God an Old/Wild God is what the being is, a loa is defined by what they are to other entities (specifically trolls). There are a wide range of beings that are considered loa, ranging from Hakkar the Soulflayer to a number of Ancients to "ascended" trolls such as Zanza.
Now that we know what Old Gods, Wild Gods, and loa are, what is G'huun. First of all, the simple fact that G'huun is worshipped by some trolls makes G'huun a loa. Is G'huun an Old God? Lore states that G'huun was inadvertently created from titan experiments on imprisoned Old Gods. It was born from Void entities and other Old Gods, so this makes G'huun a strong candidate for an Old God. Is G'huun a Wild God? By virtue of being an Old God, a parasitic Void entity, G'huun is automatically not a Wild God.
So G'huun is an Old God and a loa, but it is not a Wild God.