The Gospel of Thomas is the most "legitimate" of the non-Canon Gospels, in terms of age and how it matches with the others.
Also, it's the best one, and many believe it's the truest representation of Jesus' teachings.
It's important to define "many", you know. If *you* believe it, then you should be able to explain why you do (and if you don't, likewise). If you just refer to an anonymous "many", we can't have much of a discussion about it. I know it wasn't included in my (Roman Catholic) religious instruction.
The gospel of Thomas also seems to read much more like parts of the Tao Te Ching than any of the four "accepted" gospels, with some proverbs I recognize, but also stuff like this:
Jesus said to them, "When you make the two one, and when you
make the inside like the outside and the outside like the inside,
and the above like the below, and when you make the male and the
female one and the same, so that the male not be male nor the
female female; and when you fashion eyes in the place of an eye,
and a hand in place of a hand, and a foot in place of a foot, and
a likeness in place of a likeness; then will you enter [the
Kingdom]."
Plus things which are definitely counter to mainstream Christian teaching, like that the end times have already come, and that the dead will never be physically resurrected.
It's definitely useful to study this stuff, but from my perspective it just shows more clearly how the current bible was constructed over time out of many disparate sources.
Especially when the earliest of them wasn't written until, what was said above? 40 years after his death? (I don't study this stuff in any depth myself).
Imagine first that you're living in a time when miracles are considered fairly commonplace -- Jesus wasn't the only one thought to be working miracles at the time.
Now imagine you're in one of a few small groups of early Christians who aren't writing anything down, just trading stories back and forth. You're also actively trying to convert people, so there would be a pretty strong motivation to:
* "fix" any seeming logical holes and inconsistencies in the stories
* include more "proof" that Jesus was great and worthy of being followed
If you bend the truth a tiny bit -- well, it's for a good cause (you just got 2 new followers!), and hey, if Jesus could feed 20 people with just one loaf and one fish, that's a miracle, and he could just as easily have fed 50 or 100.
Okay, let 40 years of that go by before anyone decides the current "canon" should be written down... then it's no surprise that you have a wide diversity of tales, philosophies, theologies, all ascribed to Jesus -- such that even after centuries of punishment of death from the (now powerful) church for possessing or distributing any non-approved "heretical" version, there are still some odd versions about (entire populations of people were exterminated for heretical beliefs, after all... it couldn't have been easy to keep the variants around). The chosen few still aren't consistent with each other, but after a point were probably copied pretty faithfully, since they had become the sacred "Word of God".
I'm perfectly comfortable with people trying to dig down to figure out the most faithful representations of what Jesus' teachings might have actually been. I doubt that it's possible to know very much about what he actually said, but some kind of a philosophy can be pieced together from the various clues. It's interesting to read just like Lao Tzu and Aristotle are interesting to read (though we have much more reliable access to what those philosophers actually said -- they were literate, had literate followers, etc.).
Where I get lost is where people decide to swallow on faith the whole kit and caboodle of a personal God, worship, prayer, souls, afterlife, heaven/hell, Jesus = God, miracles = fact, and so on, using the flimsiest of reasons (like "there are things we don't understand in the universe... hence God must exist and all the rest of this follows").
*edited for mangled idiom: the expression is "kit and caboodle"; why'd I put "kitten"? Quoted below (and so preserved for all eternity) but at least I can fix it in the original....*