What did Christian authors of the 7th - 15th centuries say about the Muslim claim of Gospels being direct revelations from God?

Briefly, here are some of the claims made in the Qur’an about Jesus, the Torah or Law of Moses, the Gospel and where they came from:

“Lo! We did reveal the Torah, wherein is guidance and a light” (Surah 5:44a; 2:87).

“And We caused Jesus, son of Mary, to follow in their footsteps, confirming that which was (revealed) before him in the Torah, and We bestowed on him the Gospel wherein is guidance and a light, confirming that which was (revealed) before it in the Torah - a guidance and an admonition unto those who ward off (evil)” (Surah 5:46).

“It is He Who sent down to thee (step by step), in truth, the Book, confirming what went before it; and He sent down the Law (of Moses) and the Gospel (of Jesus) before this, as a guide to mankind, and He sent down the criterion (of judgment between right and wrong)” (Surah 3:3).

“If thou wert in doubt as to what We have revealed unto thee, then ask those who have been reading the Book from before thee: the Truth hath indeed come to thee from thy Lord: so be in no wise of those in doubt. And be not thou of those who deny the revelations of Allah, for then wert thou of the losers” (Surah 10:94-95; 16:43).

The New Testament was completed 500 years before Muhammad received the Qur’an yet Muslims claim the Bible has been corrupted, and so the Bible must be wrong. It is worth pointing out that when discrepancies in a historical document are alleged, the burden of proof rests on the newer text. On the question of authenticity and accuracy, an article I found says this:

“In conclusion, many Muslims believe that the Quran contains the literal words of Allah which have been perfectly preserved and transmitted through the ages. Muslims make this claim not based on Islamic history as I have investigated and discussed above, but purely as a statement of faith. I discovered that once the Quran was standardized, there were minor variations over time due to the various readings and transmissions of the Quran; these variations were in addition to the dots and vowel marks that were added over time. When it comes to the Injeel, it is clear from the history of its transmission, that the variations in it are more than those in the Quran. However, I was able to conclude that neither one was 100% pure, and that the variations in the Quran or the Injeel have not altered the teachings of Mohammad or Isa respectively. At this juncture I was satisfied that the word of God as revealed in the Injeel had not been altered.” Source: https://www.answering-islam.org/Authors/Farooq_Ibrahim/trustworthy3.htm

Some Muslim scholars have suggested the Injil may be the Gospel of Barnabas or Gospel of Thomas. More commonly, Muslim scholars have argued that the Injil refers to a text now lost or hopelessly corrupted. For example, Abdullah Yusuf Ali, whose translation of the Qu'ran is among the most well known in English, wrote: “The Injil (Greek, Evangel equals Gospel) spoken of by the Qur'an is not the New Testament. It is not the four Gospels now received as canonical. It is the single Gospel which, Islam teaches, was revealed to Jesus, and which he taught. Fragments of it survive in the received canonical Gospels and in some others, of which traces survive (e.g., the Gospel of Childhood or the Nativity, the Gospel of St. Barnabas)." More information here: https://en.wikipedia.org/wiki/Gospel_in_Islam

With regard to the Gospel of Barnabas, it was most likely written by a fifteenth-century European who wrote inaccurately about the life of Jesus (Isa). The original Barnabas lived during the first century and was not one of the original twelve disciples of Jesus as the Gospel of Barnabas mistakenly claims. Here is a short extract from an article on the Gospel of Barnabas:

“If the Gospel of Barnabas were written in the first century, it would have been quoted in other documents of the same time period. It is not cited, however, a single time in works of either the church fathers or Muslim clerics until the fifteenth century. Those who claim the early authorship of the Gospel of Barnabas may be referring to the Epistle of Barnabas—a first-century book, though not divinely inspired. A reading of the Gospel of Barnabas clearly shows that it was written neither in Jesus’ time nor shortly thereafter, as alleged. It contains far too many historical errors. The Gospel of Barnabas contains quotations from Dante Alighieri, references to an edict from Pope Boniface, and descriptions of feudalism. Therefore, scholars place the date of authorship around the fifteenth century.” Source: https://www.gotquestions.org/Muslim-Barnabas.html

The gospel of Thomas is a Coptic manuscript discovered in 1945 at Nag Hammadi in Egypt. This manuscript contains 114 sayings attributed to Jesus. Some of these sayings resemble sayings found in the Gospels of Matthew, Mark, Luke, and John. Other sayings were unknown until their discovery or even run counter to what is written in the four Gospels. Here is a short extract from an article about the Gospel of Thomas:

“The gospel of Thomas was not written by Jesus' disciple Thomas. The early Christian leaders universally recognized the gospel of Thomas as a forgery (and it) was rejected by the vast majority of early Christians. The gospel of Thomas contains many teachings that are in contradiction to the biblical Gospels and the rest of the New Testament... The gospel of Thomas is a Gnostic gospel, espousing a Gnostic viewpoint of Christianity. The gospel of Thomas is simply a heretical forgery, much the same as the gospel of Judas, the gospel of Mary, and the gospel of Philip.” Source: https://www.gotquestions.org/gospel-of-Thomas.html

I can find no reliable Christian source that concurs with the Islamic view that Jesus was given “the Gospel” (whatever that might be) by God. The four New Testament Gospels are accounts of the life, the teachings and the words of Jesus. They were inspired by God, just as the rest of the New Testament was inspired by God. The Jesus (Isa) of the Qur’an bears no resemblance to the Jesus of the New Testament. And that’s because Jesus was much more than a good man or a prophet. C.S. Lewis explains the Christian view of Jesus in these words:

"I am trying here to prevent anyone saying the really foolish thing that people often say about Him: I’m ready to accept Jesus as a great moral teacher, but I don’t accept his claim to be God. That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic — on the level with the man who says he is a poached egg — or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God, or else a madman or something worse. You can shut him up for a fool, you can spit at him and kill him as a demon or you can fall at his feet and call him Lord and God, but let us not come with any patronising nonsense about his being a great human teacher. He has not left that open to us. He did not intend to. ... Now it seems to me obvious that He was neither a lunatic nor a fiend: and consequently, however strange or terrifying or unlikely it may seem, I have to accept the view that He was and is God."

John of Damascus is the closest person for what you are asking for (so far) In his 1st book John Advocates for the Christian Platonic view of God that is at odds with that of Islam. John however does not directly contradict Islam in the text that I've seen.

http://www.newadvent.org/fathers/33041.htm

In his other works he is more openly polemical against Islam.

http://orthodoxinfo.com/general/stjohn_islam.aspx

https://stpeterslist.com/islam-as-a-christian-heresy-8-quotes-from-st-john-damascene-a-d-749

From that time to the present a false prophet named Mohammed has appeared in their midst. This man, after having chanced upon the Old and New Testaments and likewise, it seems, having conversed with an Arian monk, devised his own heresy. Then, having insinuated himself into the good graces of the people by a show of seeming piety, he gave out that a certain book had been sent down to him from heaven. He had set down some ridiculous compositions in this book of his and he gave it to them as an object of veneration.” Concerning Heresies

An interesting point made in later quotes the claim of Direct Revelation seems to be a strike against Islam because unlike the prophets of the Old Testament there were no witnesses to establish the account! John also does not see any passages of the Bible that could be used to foretell the coming of Mohammed (Muslims would later apply the Pentecost prophesy of the Paraclete / Comforter being sent to Mohammed because his name comes from a similar root word in Arabic).

Addendum, Christianity prior to Mohammed has objected to "Direct Revelation" concerning the Montanist movement

And he [Montanus] became beside himself, and being suddenly in a sort of frenzy and ecstasy, he raved, and began to babble and utter strange things, prophesying in a manner contrary to the constant custom of the Church handed down by tradition from the beginning.1

1. Eusebius, “16”, Ecclesiastical History, Book 5.

(Besides prophesying in a state of Ectasy, Christians objected to the fact that the Montanists spoke from the 1st person when they prophesied).

The Received New Testament also speaks against the concept of direct revelation in the following verses:

1 Corinthians 13:12 King James Version (KJV) 12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.

1 Corinthians 14:32 ► The spirits of prophets are subject to the control of prophets.

2 Corinthians 13:1 ► "Every matter must be established by the testimony of two or three witnesses."

Acts 1:8 ► But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth."

Luke 24:48 You are witnesses of these things.

John 15:27 And you also must testify, because you have been with Me from the beginning.

Acts 4:33 With great power, the apostles continued to give their testimony about the resurrection of the Lord Jesus. And abundant grace was upon them all.

Conclusion: From a Classical Christian Theological perspective the matter of direct revelation was settled centuries before Mohammed. In researching the OP it looks like there is possibility of finding something in the writings of West European authors of the Middle Ages on Mohammed based on such notions that Mohammed was demonically possessed and fulfills the role of the Christian Anti-Christ (And again from a Classical Christian perspective there is much material to work with to make such arguments!). But for the time being I may have exhausted what material can be gleaned from internet searches but will update if I find anything extra on the topic.