He doesn’t really have any secrets to hide, he would likely be happy to tell his story to anyone who asked. Here it is.
Long ago, Gunther was a fungus farmer in the village of Barbas, keeping out of the affairs of the Isle of Dread like all Mongrelmen. His only real contact with the outside world was the occasinal Troglodyte who came to barter. Gunther was almost fanatical about creating the best-tasting fungus dishes he could, and spent many hours learning from every Mongrel and Trog he could find about secret herbs and spices that they knew about. He even began taking clandestine trips to the surface to search for these plants that only grew above the surface.
One day, on one of his trips to the surface, he stumbled across the remains of an adventuring party who had been slain by a rampaging group of dinosaurs. The movement of one seeming corpse caught his attention, and he quickly made his way over to the dying alchemist. Gunther carefully cradled his head in his arms, offered him a sip of water, searched his eyes, and quietly whispered if he knew what the best way to flavor a fungus souffle was. The dying alchemist opened his eyes in confusion, beheld the hideous visage of the ugly Mongrelman, and promptly died in his arms.
While rooting through the corpses of the fallen adventurers, Gunther stumbled upon a treasure trove of information- the Alchemist’s formulae book and equipment, which coincidentally fit Gunther perfectly. Knowing that Barbas would want to split his precious treasure, but still loving his town, he returned into the caves to secretly guard his hidden village while remaining unseen in the shadows under the earth.