In Nazmir there is a placeholder scene that is supposed to show G’huun. Exact text of the placeholder scene is: “Hezrel reveals G’huun being an Old God, being experimented upon, and reveals Blood trolls want to destroy Zuldazar in order to free G’huun.”
Immediately after that, Titan Keeper Hezrel comments “Old God G’huun behind destruction of containment structure.”
I know it was speculated that he would not be a full Old God due to some vague descriptions of G’huun, but I don’t see any argument against G’huun being Old God #5.