A brief study of the origins of it shows me that nobody has been able to track down where it came from. One scholar seems to have spent considerable time in the 19th century trying to find its sources, and never could establish any authenticity to it, even as to its ancient origin.
In reading just a little of it, Section 7, Lection 69... I can see that it teaches very different ideas from the gospels Matt/Mark/Luke/John. It has ties into Zoroastrianism and Gnosticism with the concepts it puts forth.
My current position is that it is not a true gospel. Since nobody knows its origins, I will not speculate further. But I do not think that it is a valuable tool for study as far as Truth goes. I feel that the 4 gospels we have, along with the NT and other early Christian writings, paint a very consistent picture of Christ's teachings, and the Gospel of the Twelve is not congruent with that testimony.
I have studied the Bible for over 40 years. I focus on the Scripture itself, and study the surrounding writings with the same veracity. Bringing literary and historical context to Bible study.