There is no antivenin for Gila monster bite. It's similar to rattlesnake venom, but there's a lot less of it and is is not injected but oozes from glands onto the teeth. These teeth do often break off in the wound and frequently cause infection which is usually treated with antibiotics. Seems like this person likely had some sort of hypersensitivity or had other medical issues. Fatalities are rare, but complications and infection is common. Possibly he was immunocompromised.