The Gospel of Barnabas is rejected by Christians because of the contradictory statements with respect to other "accepted" gospels (Even though other accepted bibles contains some contradictions with respect to other accepted ones). It is also rejected by Muslims because of the contradictory statements with respect to the Quran.
However, there is a super surprising statement in the gospel that mentioned Muhammad PBUH and the Shahadah (almost exactly the same statement that Muhammad PBUH taught, just in a different language) in chapter 39.
There is only one God, and Muhammad is the Messenger of God.
How can Christians explain this?
Also the Gospel of Barnabas is more parallel to the Quran (example: crucifixion) than the other gospels (Not fully parallel as there are some contradictions).
There are claims about forgery, but as far as I know, there are no evidences about it also, again, since there are obvious contradictions in the gospel with respect to the Quran and the word "Muhammad" PBUH was mentioned (by the exact name) exactly (or maybe at least) 15 times.