One for you while I'm away (and what a trip it's been! It took us longer to drive to Ottawa than it takes to fly from Toronto to Australia).
What we know:
Gil Lonescale was always a bit of a snake, but Tom Crow, the medicine man of his tribe, saw potential in him and tried to untwist Gil’s path. When Tom’s other student, the slow-witted Daniel Morningstar, became Saguaro, Gil was consumed with jealousy. He went out into the desert himself, seeking power and the favor of the spirits, and got the bite of an irradiated lizard. The venom and sinister spirits he called upon transformed him into a reptile-man able to spit poison, and he swore the Gila-Master would destroy the “Chosen of the Spirits”!
Text quoted with the permission of Steve Kenson and Ad Infinitum Games, and is not intended to challenge the copyright.
I had real trouble thinking about ways to involve Gila-Master if the Mighty Saguaro isn't one of your campaign regulars. Gila-Master's origin is just so tightly bound with Saguaro, that if you don't have Saguaro, you probably don't have Gila-Master. In that sense, he's more like a model arch-foe you can reskin for your heroes to deal with. Maybe he's actually an amphibious villain (I think there might be spitting frogs, and spitting toads are thing of legend; there are definitely poisonous frogs), or aquatic, or a snake of some kind ("the Nefarious Naga!"). But this series is really about the characters as presented, so let's think about Gila-Master.
If you just want him as a one-time villain, rescue Saguaro from Gila-Master; he's holding Saguaro under the reptile house at the zoo, and some vegetarian lizards are slowly eating Saguaro alive. (There are a few mostly-vegetarian lizards, such as desert iguanas, uromastyx, and Chuckwallas. Desert iguanas are the best thematically, but they need a lot of space.)
Another option is to make up some reason for the PCs to be the Chosen Ones. Gila-Master can talk with the spirits (his occult knowledge is a reasonsto stunt Precognition, ESP, or an occasional plot-device power) ot to knock on Gila-Master, but he's strongly tied to Saguaro. He's an excellent example of an arch-foe or a nemesis, but there has to be a reason for him to interact with your players, and with other villains. Gila-Master would be an excellent part of a villain group. He would be an excellent member who causes the group to break up, later. (He is a poisonous traitor, after all.) and another mind-controller--but reptiles are relatively common.
Does he get along with Count Malocchio? They do have an occult similarity that could lead them to cooperation or conflict. (Both want the same occult artifact or ingredient, for example, or some demon or spirit for its own plans orders them to work together.) I see his lust for power as being more naked than the Count's: unlike Malocchio, he's given up the appearance of normalcy.
Gila-Master would make a great member for short-term groups. He will eventually betray them (it's in his nature--see his Qualities), but it might be interesting if the same zoo is a hiding place for Gila-Master, ConfederApe, and the Creeper. (One of the zoo staff is making money by providing hiding space to supervillains, and the ones with animal control powers naturally gravitate to him or her.) He might make a good associate for Malocchio, so long as you understood that Gila-Master would betray him--Malocchio, as nobility born and raised, would treat him like an underling, and I don't think that Gil would take that when it wasn't necessary.
His Goals and Tactics
Gila-Master wants to destroy Saguaro (and anyone else designated a "Chosen One"). He wants to prove Tom Crow wrong, and he wants to have power. The question is, what kind of power?
He isn't interested in approval or things of the lusty flesh: even if the decision to become a giant lizard was an emotional one, he has to stick with it. For Gil, that means more: more occult power, more money (people fake love if you have enough money, no matter what you look like). His current life strategy is doubling down on evil. When the spirits talk to him (and the spirits do talk to him: he is their own skinwalker), he listens.
Assuming you don't have Saguaro as a player character in your group, at first your players encounter him while he's doing things other than destroying the Chosen One: he is stealing money to get necessities; he is stealing occult items to gain the ability to do something else; he is threatening Saguaro or the ones that Saguaro loves, and Saguaro cannot respond. (Perhaps Saguaro foolishly made a bargain not to fight crime for thirty days, or not to set foot in the city for a month.)
His initial attempts to destroy Saguaro are physical and head-on. He gradually expands that. If he cannot destroy Saguaro himself, perhaps he can destroy Saguaro's reputation, or Saguaro's loved ones. He discovers (possibly through the spirits) that the PCs are also Chosen Ones...perhaps everyone with super powers is a chosen one. Gila-Master is large; he can hate multitudes.
- The same zoo is the hiding place for the Gila-Master (the reptile house), the Creeper (deep in the elephant enclosure) and ConfederApe (the zookeeper is making a little extra money hiding out supervillains, and the ones with animal control powers naturally gravitate to him or her.) Finding out about each other, they decide to cooperate, for a while. The PCs must deal with the three villains (who cannot agree on a group name: "The Manimals!" "The Wild Kingdom!" "Biospear!"). When things start to go bad, Gila-Master leaves his two associates in the lurch.
- Gila-Master has kidnapped the Mighty Saguaro, and is holding him in the tunnels underneath the reptile house. Saguaro is unconscious through a convenient spell he has learned (his poison took down Saguaro by chance). He is forcing Tom Crow to watch as desert lizards nip off the spines of the Chosen One. ("Thus does the student beat the master!") But Crow has managed to get a last warning out to the PCs, who must find and rescue Saguaro. As a delaying tactic, Gila-Master will free poisonous snakes and Komodo dragons to threaten the zoo visitors, and eventually, leaves the unconscious Saguaro to deal with the PCs.
- One of the PCs finds his strength is wildly out of control. (This can be a PC who is absent for the session, or something like the Uncontrollable limit added just for the session.) The normal treatment for this ailment is derived from snake venom, but the super-powered nature of the PC means that normal treatments aren't going to work. They need something stronger. They need venom from Gila-Master. The PC can still function, but because it will take time to refine the venom, they need to get the Gila-Master now. If he's in prison, he makes a deal to get out. "I'll have your friend's life if you free me." If he's free already, they have to track him down. Simply finding him, fighting him, and capturing the venom might work, but they will be better off to get Gila-Master's cooperation, because they might need a lot of venum.
- With the aid of a rogue chemist, Gila-Master discovers that his venom can also be used to make a hallucinogenic drug, which he markets and calls (appropriately enough) Venom. He markets it illegally, of course, and eventually has to deal with his partners. And Venom is highly addictive. Further, the drug transfers a tiny bit of the vril or the power of the user to Gila-Master. This new drug is hitting the streets and doing terrible damage; can the PCs find the ultimate source and shut it down?