I wish I had time to address all of your questions, but I have to get the revisions of the Thirteenth Apostle to my publisher by Friday. So briefly:
1. The gem is green jasper, like the Brummer gem that Josè mentioned. The lion-headed god is the same image as found on the Brummer gem, and on many many gems from the ancient world with names of the astral god including IAO, Abrasax, Chnoubis, Michael, etc. On the Judas gem, he holds a medusa-head on display. So the gem is likely a protective or aggressive magic gem. There are two cartouches with identical inscriptions on the front. They appear to me to be palindromes. Within them are hidden anagrams for the names Michael and Elieli, both angels associated with Ialdabaoth in Gnostic traditions. There are also a series of magical characters which represent various stellar and planetary signs. On the back, centered and alone appears the inscription "IOUDAS". The iconography of the god on the front suggests a 1st or 2nd c. date from a Greco-Egyptian workshop. The inscriptions on front and back are made by the same hand, so Judas was not added at a later date by someone else. I imagine that it was either mounted in a ring or in a pendant, although the mount does not survive as far as I know.
2. The Mithras leoncephaline god is another example of this same astral lord that I have been talking about. The best books on the subject are Howard Jackson, The Lion Becomes Man, and David Ulansey, The Origins of the Mithraic Mysteries. This astral lord goes by many names among the ancient people. Some Gnostics called him Ialdabaoth, Saklas, Samael, Nebruel, Michael, Elieli, and Judas. Other ancient people called him IAO, Chnoubis, Abrasax. We don't know the name of the Mithraic version, although he seems to have been a very fierce and terrifying god. This astral god was feared by the ancient people because he controlled the universe. He ruled it and our fates. He usually has a leonine head or a cock-head, solar rays, and also serpentine form. He is the pole serpent, the one who controls (or is) the axis of the universe. My student Franklin Trammell is making a complete study of the pole serpent now for his dissertation, so perhaps I can encourage him to write a short guest post on the subject.
3. I am so discouraged that the hero Judas has made it into the Sunday school catechisms already. This was one of my main fears, and why I so quickly published my book. The NGS is so influencial. People take the Society's claims as true. This business if very sad to me because the public has been misled, and I see no honest attempt to correct this misperception. The more I study this document, the more evidence amasses that Judas remains a demon. The Gnostics took the canonical gospels at face value - that Satan entered him (John 14:27). Who was Satan in the Christian tradition? He was the "ruler of the world" according to the Gospel of John. It is not such a big leap for the Gnostics to have called this ruler Samael (Satan's alternative name in Judaism) and Ialdabaoth (the magical name for the astral ruler), and to have said that this is the figure that Judas became. This is just ancient logic. Nothing more.