The Gospels are actually OT ( written before the church was founded)
How so? Most people would say the church was founded at Pentecost.
That is why the gospels are technically OT . They cover the period of time when Jesus lived and ministered, died and rose again.
Jesus lived under the law in the Ot period of sacrifices etc.
The church was born at Pentecost and that was after the Ascension.
That is the start of the NT writings which were instructional/correctional letters to the church and of course the final prophecy of John to the church about the end of time.