The Gospel of Barnabas is a fake work. The English version was translated in the early 1900's from the Italian version. The Italian translation could be as old as 1400 AD. No other copies exist, and there are no references of it from anyone before the 1500's AD.

This should not be confused with the Epistle of Barnabas which dates between 70 and 135 AD. The epistle may or may not have been written by the Barnabas of Acts, but it is quoted by early church fathers like Clement of Alexandria, who about 170 AD, stated the epistle was authentic. the teachings in the Epistle of Barnabas agree completely with the Bible.

Contradictions 

1. The Gospel of Barnabas says Jesus stated "I am not the Messiah" (sec. 42,48) which contradicts both the Bible (Matthew 16) and Quran (Sura 5). 
2. It calls Paul apostate, circumcision is necessary for Salvation (Sec. 23) 
3. Jesus did not die on the cross, Judas did in his place (Sec 217), and Jesus is not God incarnate. 
4. "Jesus was born when Pilate was governor" (Sec 3) 
5. "Jesus sailed to Nazareth" which of course has no seaport. (Sec 20)
6. Mary brought forth her son without pain. (Sec 3)  This is contradicted by the Quran (19:23).
7. The Quran condemns eating pork but "Barnabas" says "that which entereth into the man defileth not the man, but that which cometh out of the man defileth the man" (32).
8.  Daniel was taken captive by Nebuchadnezer while he was 2 years old.(Sec. 80)
9. And many others.

It does however teach the wise men worshiped Jesus (Sec. 6) which agrees with the Bible but contradicts the Quran.

Other links about the Gospel of Barnabas:

http://www.answering-islam.org/Barnabas/