As you probably already know, the four gospels in the Christian canon (Matthew, Mark, Luke, and John) aren't the only ones penned about Jesus Christ. Perhaps the most well known of these other gospels is the 'Gospel of Thomas'. In fact, Robert Funk, representing the Jesus Seminar, published a book called, "The Five Gospels" in 1996 which offered a fresh look at Christ's life and used the 'Gospel of Thomas' right along with the canonical four.
If you're like me, when you first hear the word 'gospel' you picture the gospels you grew up reading. You picture a sustained narrative starting with an infancy narrative (at least in the case of Matthew and Luke), followed by Christ's being anointed by the Holy Spirit at His baptism, an earthly ministry which includes teaching about the Kingdom of God and miraculous signs, the passion week, and finally, the resurrection/ascension. You might be surprised to learn that, when compared with the four gospels found in the Bible, the non-canonical (the term I use to describe what are commonly called "gnostic" gospels) gospels have many differences to ours (e.g. time of writing; period of Jesus' life described; number of extant manuscripts; acceptance by early Christians; context within Palestinian Judaism; nature of Jesus' teaching).
I would be irresponsible if I tried to explain all of the non-canonical gospels to you because, frankly, I'm not qualified. The world of gospel studies is one of the most intricate, confusing, and intimidating subjects you could ever endeavour to learn about. However, if you are interested I highly recommend you read the source texts which can be found on Early Christian Writings (in fact, reading them for yourself is the best thing you can do if you're interested in them). The site isn't a conservative one, but it is a great tool for studying source texts.
If you are one of those interested in reading non-canonical gospels, allow me to offer you a couple of tips. First, as you enter this strange world of gospel studies, remember that scholars will sometimes talk about hypothetical documents as if they are real. For example, you can go to the above site and read the document known as 'Q' (from the German word for source, 'Quelle'). However, Q doesn't really exist, it's a hypothetical source document.
To make a long story short, Q is the material common between Matthew and Luke, but not found in Mark. If you look at the visual aid below, Q would be represented by the blue "Double Tradition". Scholars theorize that the only way Matthew and Luke could have material so similar, is if they borrowed from a common (and probably written) source - hence, Q. There is no surviving evidence (such as existing manuscripts) that Q ever existed, and yet in the world of NT studies, it might as well be as real as the nose on your face. To be fair, there is nothing unreasonable about Q, in fact, I too believe that some source (whether written or oral) like Q did exist.
I would also like to offer you some categories to help you think about the non-canonical gospels. Some of the non-canonical gospels often "fill in the gaps" of our gospels. For example, there is a period of time between Jesus' resurrection and ascension in which there is very little of his teaching recorded, some of the non-canonical gospels (e.g. Epistula Apostolorum) attempt to fill this gap (I would call this category 'Post Resurrection Revelation'). There are also gospels which are 'Pre-Infancy Narratives' which discuss, to put it generally, the time before our gospels began (e.g. Infancy Gospel of James; a.k.a. Protevangelium of James). Another category (again, as I would describe them) is the 'Post Infancy Narrative' (e.g. Infancy Gospel of Thomas) which describes Jesus' boyhood (filling in the gap between Jesus' birth and ministry). The Gospel of Thomas actually fits into the category of a 'Sayings Gospel'. If you were to read it you would think to yourself, "This reads much like the book of Proverbs.". A 'Sayings Gospel' is simply a collection of Jesus' sayings, all strung together one after the other without any contextual background information such as an audience, setting, or location (i.e. sitz im leben). Finally, there is the category of Passion Narrative (e.g. Gospel of Peter), which describes Jesus' crucifixion.
In studying the non-canoncial gospels, I have developed a much deeper appreciation and trust for Matthew, Mark, Luke, and John. Comparing and contrasting our gospels with the non-canonical gospels provides a priceless perspective, and helps us better understand the New Testament. The trustworthiness of the canonical gospels, evidenced by their time of writing, straightforward content, early acceptance, and textual preservation is amazing. I feel fully confident learning about my Savior from these four remarkable documents.