Which New Testament books supposedly pre-date Christ?
I do understand that the Dead Sea Scrolls actually lend support to the inclusion of the deuterocanonycals in the Old Testament (which is what the earliest Christian records prove Catholics have done for 1700 years, and all Christians did until the protestants edited the bible in the1500s).
None, There were other books in the Dead sea scrolls.
Perhaps the reference is to the 'Gospel of Isaiah' (i.e., the Book of the Prophet Isaiah)...
Isaiah lived around 700 BC, but a lot of his prophecies provide some very good descriptions of the LORD Jesus Christ, and HIS work of Salvation. Thus the reference: 'Gospel of Isaiah'.
It is no wonder that the LORD frequently referenced Isaiah's (aka, Esaias') writings in HIS ministry (HE probably wanted us to read this book closely).
I believe the Dead Sea Scrolls included almost the entire book of Isaiah, and confirmed that it matched our current text.
But truth be known, I suppose the entire Old Testament pointed to Christ:
Luke 24:27 And beginning at Moses and all the prophets, HE [Jesus] expounded unto them in all the scriptures the things concerning himself.
John 5:39 [Jesus said] Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.