While there's been a large amount of discussion and debate amongst scholars and theologians over authorship issues, I would say that the real controversy for the various churches themselves has been less about how the books were physically written (or by whom), than about the degree to which they should (or shouldn't) be considered the literal, inerrant 'Word of God.'
You might want to x-post this to /r/religion, for further insight.
To answer the question as to how has Christianity reacted to this, the objections that Ehrman and others have brought up are certainly not new. Faustus the Manichean was the very first person who raised questions about the genuineness of the authors in the 400's, to which Augustine raised questions about holding double standards. (Which is interesting, for several centuries no one questioned it that we have record of)
"Why does no one doubt the genuineness of the books attributed to Hippocrates? Because there is a succession of testimonies to the books from the time of Hippocrates to the present day, which makes it unreasonable either now or hereafter to have any doubt on the subject. How do we know the authorship of the works of Plato, Aristotle, Cicero, Varro, and other similar writers, but by the unbroken chain of evidence?”
Against Faustus 33.6 (~AD 400)
Modern criticism relies heavily on internal tests, which I think can be somewhat subjective and questionable. An example would be that Paul's theology can come across differently in Ephesians than it does in Romans. We know he wrote Romans, so he couldn't have wrote Ephesians. There's other objections, like the supposed tension between Matthew and John...etc. This is highly debatable (the theology in Romans and Ephesians isn't really that different, neither is the Christology between Matthew and John). I think looking at the external evidence is much less subjective.
What Christians respond with, and I think rightly so, is they would rely more on external test to determine genuineness of authorship.
In the case of the four gospels, each of the following writers and documents either names or describes the authors of one or more of the Gospels:
Furthermore, many early writers make use of the Gospels without naming or describing their authors. This evidence takes us back even earlier than the evidence of attribution. For these authors to make use of the Gospels as authoritative sources means that they expected their audience to recognize their quotations and allusions and to accept them as authentic. Several examples could be easily cited from Ignatius, Polycarp, Basilides (a gnostic heretic), Clement of Rome and so on. The four Gospels and Acts are used copiously by the early church fathers, taking us back to within the lifetime of the Apostle John himself.
Examples of educated Christians who have responded to the claims of genuineness of the gospels throughout the centuries are Augustine, Aquinas (IIRC), Calvin, William Paley, Samuel Tregelles, Andrews Norton, Richard Watson, B.F. Westcott, James Gray and many others. A nice, brief rebuttal to some of the claims of Ehrman in terms of authorship of the gospels is summed up rather well in this video. https://www.youtube.com/watch?v=V2kRn6y_qOE&index=15&list=PL52A88324AB3C3E7A
Could you perhaps provide links/sources to these modern Biblical scholars?
If by sects we mean denominations, the reaction has either been to accept the consensus of scholars, reject it, or take no position. It varies from one sect to the next and has a lot to do with how the individual sect views itself in the context of the life and ministry of Jesus Christ.
It is worth pointing out that there is still a string of Christian theologians and Biblical scholars who still hold to the traditional Pauline authorship of the disputed epistles. So claiming that
Modern Biblical scholarship has come to the conclusion that many letters and books in both the New and Old Testament were not written by the authors that they have been traditionally ascribed to.
is not entirely true. I will concede that Evangelical and Conservative Christians (myself included) have a vested interest in disproving the inauthenticity of those authorships.