Read and compare. I've done some. For example, many liberal churches love the Gnostic gospel of Thomas yet have issues with St Paul.
Let's look at a passage from the supposed Thomas text and compare with Paul's epistle:
Simon Peter said to them, “Let Mary leave us, for women are not worthy of life.” Jesus said, “I myself shall lead her in order to make her male, so that she too may become a living spirit resembling you males. For every woman who will make herself male will enter the kingdom of heaven.” (Thomas 114)
For as many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. And if you are Christ’s, then you are Abraham’s seed, and heirs according to the promise. (Galatians 3:27-29)
Add to the fact most of these 'lost gospels' and NT apocrypha books were later extant developments and were not quoted in the sub-apostolic era nor by the early church fathers.
Find me a sermon by Justin Martyr or Polycarp or even Barnabas where they quote 'lost gospels' and I might consider the passage.