Question: Which apocryphal Gospel is the most important for reconstructing the life of the historical Jesus?
AP: The most significant of them is, without a doubt, the Gospel of Thomas, which doesn't really pay much attention to the facts of Jesus' life, rather just his spiritual judgments. Basically, it's like a mystical Gospel in that the only thing that matters are the salvation of an individual and the spiritual fusion with Jesus the Revealer.
Some scholars believe this Gospel holds as much value as Matthew, Mark, Luke, and John. In my opinion, however, this position is somewhat exaggerated because the text that we have today was certainly rewritten or edited around the middle of the second century. The truth is scholars do not agree about which parts of this Gospel are old and which are not.
In spite of this, I think we can at least some sayings of Jesus contained therein are ancient.
TWH: From what I have read on the subject, most scholars in this field believe or at least give greater credence to the Gospel of Thomas. Whatever parallels there are between the Gospel of Thomas and the canonical Gospels should be given our attention when thinking about Jesus' life. After all, what we find in those four Gospels accurately reflect the life and ministry of Jesus. Thus, parallel content in the Gospel of Thomas is by default reflective of Jesus' life in so much as they communicate what we find in those inspired by the Holy Spirit. Is it possible that something found within the Gospel of Thomas, though not recorded in the canonical Gospels, actually communicates something that actually occurred either in Jesus' life or teaching. Sure, that is possible. Nevertheless, such searches are vain. I recommend concentrating one's efforts entirely on the study of Matthew, Mark, Luke, and John, for numerous reasons, but especially if such a person wishes to know who Jesus is and what he did during his life and ministry.