There are around 20, possibly more, Lost Gospels. Some have been re-discovered in part. Some have been saved through the centuries in part. Others were lost to the rigors of time or were destroyed by the church. None of these gospels is included in the Bible, though parts of some appear to have been mixed in with the existing gospels.
There is a lot of controversy surrounding the Lost Gospels. Some of this controversy involves their known content. Some of the controversy includes opinions of them over the years and some stems from why they were not included in the Bible. The official position of the church is that some of them were of unknown origin, so they could not be included and some were heretical or written by heretics. There were also almost certainly some that could not be included because they were unknown to the church. People who believe that the biblical canonization of the gospels was a matter of convenience for the church will tell you that some gospels were excluded to suit the needs of the church. In other words, anything that went against what the church was trying to teach was considered heretical and excluded, regardless of its authenticity or the veracity of its author.
Among the hypothetical and Lost Gospels is the Gospel of Mary, which may have been excluded because it placed a woman (Mary) as the leading disciple of Christ. Giving a woman such importance is clearly not good for the Bible. There is also the Pre-Markan Passion, which describes the last days of Jesus on Earth. Then, there is the Gospel of Peter, which describes the resurrection. There are many that no one is quite sure what they said if they even existed at all. Then, there are those that are the most controversial and well known.
No one is sure what was included in the Secret Gospel of Mark. The only reason we know it existed is that Clement of Alexandria wrote a letter about it. In it, he extolled the work of Mark but claimed it was being misinterpreted and had become mixed with “utterly shameless lies” by Carpocrates. Of course, he does not fully explain how he knows them to be lies. In fact, it kind of sounds like he just did not like what Carpocrates had to say about the Gospel of Mark. The gospel was supposedly kept in Alexandria at the time and only viewed by those who were allowed to read the “Secret Gospels.” Carpocrates got his hands on a copy somehow and managed to put Clement in a rage. Unfortunately, no one knows where this gospel is now.
The most controversial Lost Gospel is the Gospel of Eve. As the Gospel of Mark, no one knows if a copy even exists anymore. The only evidence we have for it is in someone else’s writings. In the existing writings regarding the Gospel of Eve, it is said that a group of Gnostics had used the gospel as an excuse to use birth control (“pulling out”) and swallow semen. They also enjoyed sex with multiple partners. The only parts of the gospel that remain are those copied into the papers and they are minimal. None of it hints at encouraging carnal lust. However, the church and Epiphanius (the man who wrote about it) had access to the entire text and they obviously thought it was seedy. They destroyed every copy they could find.
Chances are we are never going to know what the bible would have looked like if all of the gospels were included. However, there is no accounting for interpretation. They could probably have scenes of carnal lust and heretical ideas all through them and still be interpreted differently by different people. That is the nature of ambiguous texts. Though, it would have been nice to see the Gospel of Eve. It would be interesting to know what was so bad it was worth burning. Maybe it was the rough draft of Harry Potter.