Many people are familiar with the canonical gospels; that is, familiar with the type of material they contain. The canonical gospels exist to convey accounts of the life and teachings of Jesus. They have a biography-style presentation. Matthew and Luke start just before Jesus' birth and quickly move on to Jesus' entry onto the public stage; Mark and John begin with Jesus' debut as a public figure at his baptism. From there, all four canonical gospels relate a series of teachings and events. Large parts of the narrative are event-driven, particularly the confrontation with the religious and political powers, a trial on capital charges, and an execution recounted in some detail including Jesus' death and burial. All four continue with an empty tomb and the announcement of Jesus' resurrection; three continue with additional events past that point. When we hear the word "gospel", we therefore tend to think of that type of document: a narrative of Jesus' life and teachings recounted in the form of biography.
A little-known, little-acknowledged fact about the "alternative" gospels is that many of them are not this sort of document at all. The best of the lot are probably the (Coptic) Gospel of Thomas, which is a collection of sayings attributed to Jesus, and the Gospel of Peter which does follow a narrative framework. These documents have certain problems, but at least they intend to recount the life or teachings of Jesus.
One other gospel does intend to give a sort of account of part of Jesus' life. The Infancy Gospel of Thomas is supposed to be a retelling of Jesus' childhood. It could nearly be subtitled, "Bad-tempered child with superpowers terrorizes village" -- at least for the first half. By the end, he has learned to use his powers for good instead of evil, and it finishes with "the boy Jesus at the Temple" account known to us from Luke's gospel. Quotes from bad tempered little Jesus include, "You godless, brainless moron" (right before he strikes another child dead) and "I taunted you! For I know that you are amazed by little things and have minuscule minds." The people of his hometown are in awe of him and his many miracles. An interesting feature comes to light when studying the text: the part borrowed from Luke contains the only mentions of events occurring in a specific geographical place (Jerusalem) and the only mention of the name of his mother, Mary. It is also the scene with a noticeably stronger Jewish context: we see the Pharisees and the Temple, along with the Feast of Passover, here and only here in the narrative. I find it interesting that a number of tangible and realistic supporting details are found only in the part that is borrowed from the canonical gospel of Luke.
Of the other well-known alternative "gospels", most show relatively little interest in giving a biographical account of Jesus' life or in giving a collection of his sayings. Based on what they actually contain, these other gospels may not have been intended to convey accounts of the life and teachings of Jesus.
- The Gospel of the Savior briefly recounts a couple of events from a single night of Jesus' life. The text never identifies Jesus by name. The events recounted are part of the Last Supper and the prayer afterward. In keeping with the non-geographical nature of the actions recorded in the text, the trip to Gethsemane from the canonical gospels is replaced here with a vision of heaven, where the prayer occurs before the Father's throne. Those are the only events from the life of Jesus that are recounted. A good section of the text consists of the main character -- presumably Jesus -- leading a responsory prayer largely centered on himself and his importance as their leader. The responsory prayer looks like an excerpt from an early Christian worship service; that section may be of more interest in the field of the history of worship than in the events of the life of Jesus.
- The Gospel of Mary, like the Gospel of the Savior, never identifies Jesus by name; but here the unnamed "Savior" is not the central character. Granted, again we are working with fragmentary pieces of surviving text, but the surviving pieces mainly consist of a vision that Mary is supposed to have seen. No events from the life of Jesus are recounted. In certain places there are some sayings attributed to the unnamed Savior, which can be divided into two categories. Many of the things attributed to the Savior are generically applicable known sayings of Jesus; "he who has ears let him hear" is used twice within the space of a few verses, and the variant "he who has a mind to understand, let him understand" makes an appearance too. In between such stock and generic phrases from the canonical gospels, the Savior's other sayings sound as though they were taken from Greek philosophy, such as "the nature of matter is resolved into the roots of its nature alone."
- The Gospel of Philip may possibly recount Jesus' baptism, though even that is uncertain because the text is only partially complete. That is the only event from the life of Jesus that may have been recounted in the way that we would have expected from the canonical gospels. The Gospel of Philip consists of more general discussion of religion and philosophy from its own perspective, and is not particularly centered on Jesus. In the surviving text, the phrase "bridal chamber" appears more often than the name "Jesus". As a point of interest, the author of the Gospel of Philip quotes Matthew, John, 1 Corinthians, and 1 Peter, expecting the readers to be familiar with them and to consider them authoritative.
- The Gospel of Truth is fairly long as non-canonical gospels go; it is roughly 40% of the length of the Gospel of Mark, or 1/4 of the length of the Gospel of Luke. The name "Jesus" occurs a mere four times in the translation I've found. It does not give an account of any events in the life of Jesus. It is largely a theological interpretation, not what we would think of as a "gospel."
- The Protoevangelium of James mostly follows the story of Mary in the years leading up to Jesus' birth. It ends shortly after Jesus' birth and the visit of the astrologers. As most of the narrative happens before his birth, its purpose is not to record the life of Jesus.
I've had to rein myself in, to make myself stop here. The measurable differences in quality between the canonical gospels and the non-canonical gospels are many, and the differences run deep. The non-canonical gospels are generally shorter, generally later, generally less Jewish, generally have less context as far as place and time, and are often less interested in recording the life of Jesus. For some of them, I think the appropriate genre is not "gospel" but "fan fiction". For others, I think the appropriate genre is not "gospel" but "theological interpretation". Some never identify identify Jesus by name in the surviving text. One is a sayings-only collection without enough background on the conversations from which the sayings are taken.
I'm not saying the non-canonical documents are without any merit; I have found points of interest in them. I may yet do a write-up on my favorite parts of the non-canonical gospels. But I am saying that even the whole collection of them together tells you measurably, objectively less about the historical Jesus than, say, the Gospel of Luke by itself.