The short answer is, the Gnostic Gospels are interesting ancient documents that tell us a lot about the people who wrote them and not a lot about Jesus.
For all those wondering what the heck the Gnostic Gospels are, they are books written from about AD150 through to AD400 that tell a version of things Jesus said and did. You’ll often hear news stories about Church conspiracies to hide these books because they don’t support their preferred version of Christianity.
The truth is the early Christians knew there were lots of things written about Jesus and that not all of it could be trusted (see 1 John 4:1-2). So, how did the early church work out which writings they would keep reading and which ones they wouldn’t? The main question they asked was whether the book said the same things that they had heard the apostles say. There wasn’t a secret committee that decided to keep some books and hide the others. The books that we have in the New Testament are the ones that the early churches were reading and keeping.
So there wasn’t really an official decision about whether to include the Gnostic Gospels in the New Testament. Over time it was recognised that these books weren’t being read by the churches, so they were never included in the New Testament.