It sounds like our Muslim friend is trying to tell us that the author of the Gospel of Mark had no idea what the Lord Jesus meant by "Gospel," because here is what Mark writes as his very first verse:
1 The beginning of the gospel of Jesus Christ, the Son of God.
2 As it is written in Isaiah the prophet:
Look, I am sending My messenger ahead of You,
who will prepare Your way.
3 A voice of one crying out in the wilderness:
Prepare the way for the Lord;
make His paths straight!
But following this Muslim logic, this is the way that the first verse of the Gospel of Mark should have been translated (emphasis mine):
The beginning of the Book of Jesus Christ, the Son of God.
Or, if Muslims prefer:
The beginning of the Injil of Jesus Christ, the S…