In the later half of the second century, Tatian, an Assyrian Christian who was a pupil of Justin Martyr in Rome, created a Gospel harmony from the four Gospels. He combined the four Gospels into a single narrative. And the harmonized version of the Gospels was called the Diatessaron. The Diatessaron became popular and even the standard Gospel within the Syriac Christianity.
Considering Muhammad’s proximity with Syriac Christians, it was plausible that Muhammad knew about the Diatessaron, and hence it could explain why in the Koran the Christian Gospels were thought to be of a single text. Although, the argument might not be that straightforward, since the Diatessaron had been the standard Gospel within the Syriac Christianity only for two centuries before finally being replaced by the Peshitta version in the fifth century which contained the four Gospels, and hence it predated Muhammad’s resurgence in the sixth century. But at least it offers a hypothesis on why Muhammad thought that Allah gave Jesus a book named the Gospel.