1. It contradicts authentic, divinely inspired revelation (Isa. 8:20; Gal. 1:8; 1 Thess. 5:21; Acts 17:11). The early church knew about this writing (Irenaeus mentions it in his book Against Heresies), easily recognized it as spurious, and therefore left it out of the canon of Scripture.
2. It did not even come on the scene until the second century long after the real Judas (Judas Iscariot) hung himself (Matt. 27:5). It was clearly an attempt by someone (a Gnostic) to infilitrate the church with false teaching. Even National Geographic states that the only surviving manuscript copy we have dates to the third or fourth century.
Below are some articles that will give you additional help with this issue.
"A Brief Overview of the Gospel of Judas" by Pat Zukeran
"Responding to the Gospel of Judas" by R. Albert Mohler Jr.
"Nag Hammadi Gospels" by Dr. Norman Geisler
"What is the Gospel of Thomas?" by Dr. Norman Geisler
"Should Additional Gospels Have Been Included in Our Bibles?" by John Ankerberg Ministry
*Gnosticism was a philosophy that developed over several centuries before Christ. Some of its core teachings are:
1. All matter is evil, and the non-material, spirit-realm is good.