They were all written at a later time Taimi, but that has nothing to do with their inclusion in the bible. There was and remains a huge debate about which gospels became canonical.
There are layers of gospels, as there are of any religious texts anywhere in the world - some more or less accepted that others.
Personally, I think they are mostly crap, because they include such a shallow layer of information for anyone serious about knowledge. But the Christian gospels are more about faith than knowledge. There are many references to insights, as there are in all the Abrahamic scriptures, and for someone who has been able to progress on the path, some of these insights are appropriate, and even useful. But in the main, they are all about swaying us on the basis of emotion and faith (which means believing without knowing why).
By all means trawl through these texts, and find some helpful insights, but if you think to use them as a basis of the journey into truth, you will be deluded.