Here is what Irenaeus says about The Gospel of Judas in his Against Heresies, 1.31.1:

Others again declare that Cain derived his being from the Power above, and acknowledge that Esau, Korah, the Sodomites, and all such persons, are related to themselves. On this account, they add, they have been assailed by the Creator, yet no one of them has suffered injury. For Sophia was in the habit of carrying off that which belonged to her from them to herself. They declare that Judas the traitor was thoroughly acquainted with these things, and that he alone, knowing the truth as no others did, accomplished the mystery of the betrayal; by him all things, both earthly and heavenly, were thus thrown into confusion. They produce a fictitious history of this kind, which they style the Gospel of Judas.

We can be very confident that the text Irenaeus is talking about and the GJ that has recently been published are the same text. Which means that this form of (Sethian? Cainite?) gnosticism, which clearly wants to find its origins in Jesus, is prior to 180 AD (when Irenaeus wrote his book). Which means we should date this text to something like at least 150 AD (giving it time to circulate). We’re guessing on precise dates: Irenaeus may have had to rebut it because it was the most recent rage or he may have rebutted it because it had been around long enough to deserve his attention.
Ben Witherington suggested this week that GJ might not be the text Irenaeus is referring to; you can make up your mind on the basis of this text I’ve quoted above and the summaries I offer this week. Here’s what I wrote to Ben.
Ben,
I agree with all your comments, except that I think there is pretty solid evidence the GJ is the text mentioned by Irenaeus (there is no evidence he actually read it, but his summary fits the GJ). Wurst’s chp in the NG volume is pretty good.
Here’s what Irenaeus says:
1. Judas was acquainted with the tradition of opponents of the tradition being right.
2. Judas knows about this Wisdom.
3. Judas alone was acquainted with these things perfectly fits what GJ says about Judas; several times Jesus takes him aside as one special.
4. Judas did accomplish the mystery of the betrayal in GJ.
5. The dissolution of the earthly and heavenly is what Scene 3 is about in GJ.
There seems to be some conviction this was originally written in Greek, but I don’t know why.