A gospel is a writing that describes the life, ministry, death, burial, and resurrection of Jesus.
The "gospel" of Barnabas contradicts the Gospels and Bible.
It wasn't even written by the apostle Barnabas, so it in and of itself is forgery (trying to make his [whomever wrote it] gospel appear legit by producing it under the name Barnabas) and it is a false gospel.
Also if Barnabas actually wrote a gospel, it would probably appear in the NT of the Bible or the Apocrypha.
2 John 1:7-11
For many deceivers have gone out into the world, those who do not confess the coming of Jesus Christ in the flesh. Such a one is the deceiver and the antichrist. Watch yourselves, so that you may not lose what we have worked for, but may win a full reward. Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son. If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, for whoever greets him takes part in his wicked works.
Romans 16:17
I appeal to you, brothers, to watch out for those who cause divisions and create obstacles contrary to the doctrine that you have been taught; avoid them.