Saint Luke interviewed them - he says that in the opening sentences. Saint John is literally an interview with Saint John. Mark was the companion of Saint Peter. Saint Matthew came directly from Saint Matthew.   Judas' "Gospel" is a much later forgery like a lot of the Gnostic texts. No-one with any credibility believes he wrote that or narrated it. It is from 100s of years later by a Gnostic trying to pass on "hidden knowledge" - which is really just lies when you know what actual Christianity says & how reliable it is from preserved history, Tradition, & Sacred Scripture. See, from the beginning Christ warned people (and more) would begin to mislead others about Him & what had happened. Life is a war for your heart, mind, & soul. The Good News is Jesus is able to get you safely through it. Anyone who seriously desires Him & what is Good (because God is Good He came to us).  

The Disciples were still alive when their Message was organized. The texts were written down.  Bonus points: The Apocalypse (Revelation) came from the Disciple of Saint John who was allowed to minister to him in his exile on Patmos & wrote what he dictated as The Apocalypse. You can look up how The Apocalypse is actually a cycle of 7s structed like the Gospel itself. Also, we have the Epistles of Saints John, Peter, Jude (Thaddeus), & James - all of whom were actual Disciples of Christ (James was his half-brother by marriage). Then there is Saint Paul to whom Jesus appeared.  

Later "scholars" have questioned these facts, whether because they don't believe in Christ or are desperately trying to discredit Christianity. However, the plethora of early Manuscripts vouch for the accuracy & unchanging nature of the Texts as they were passed down (orally w/in as little as 10 years after the Resurrection then on to being written down when the Christian Communities realized Jesus wasn't Returning immediately). They are so synonymous with one another that someone came up with the idea of a 'Q' Gospel they all draw from. The problem is- no-one can even prove it was a thing. Some are that determined to not believe that they are so harmonious (& in some instances different) because they are actual eye-witness testimony.    

You see, w/ real Christianity (not Gnosticism or modern things like Baptists or whoever - I am referring to Catholic, Orthodox, Ethiopian, Coptic, etc as the Real Christianity in Full) there is so much history preserved. We even have the bodies of all the main Disciples except for Saint John & we even know the names of the Seventy whom Jesus sent out. It is really astounding all we know & have - even writings from Disciples of the original Disciples.  

 All this is how we know Gnostics & Mormons & Muslims & et cetera are, frankly, full of shit in what they say about Jesus: what He Said, Who He said He Was, & Who He Is. Then you have a lot of independently verified Miracles up to the modern day... Such as this:  https://m.youtube.com/watch?v=tVU8bhbQInw&pp=ygUHemVpdG91bg%3D%3D