mehrosh, I have come across this sort of debate before in islam.com. This is nothing more than medieval forgery. That a Muslim would bring this argument up, over and over again, I can only admire Muslim persistence of clear propaganda agenda, but not their illogical argument.
I have read it before, and there are too many clues that are left in the text that indicate it is clearly medieval pseudepigrapha literature.
There is a clear indication that Barnabas and Paul were friends and travelling companions, but in this gospel it showed strong denunication against Paul.
But that's not what make it a forgery.
It is clear in the bible that he was really named Joseph, but the apostles named him Barnabas after Jesus' ascension (Acts 4:6). But in the gospel of Barnabas, he was among Jesus' apostles several years before his ascension and that Jesus had called him by the name "Barnabas" instead of "Joseph", is not really credible.
There are discrepencies found in the so-called gospel that indicated that it is fake.
The gospel says that the jubilee happened every 100 years, but throughout the Jewish history, and recorded in Leviticus 25.11, jubilee only happened every 50 years.
Only Pope Boniface (I don't which Boniface; sorry I don't keep track of number), around 1300, had decreed a 100-year jubilee, before Pope Clemens (don't know his number too) had reverted the jubilee back to 50-year. So this gospel could be written at this period.
The gospel also mention some objects that didn't exist at that period, and could only have come from medieval time, such as the wooden wine barrel or cask. Such thing is medieval invention. Wine were store in ceramic jars or wineskins in ancient times.
The other things is that the gospel allude to feudal towns, when Barnabas speaks of Larazus and his sisters.
The climate is also wrong for this region in Palestine, and wrong about the geography too.
I may not know as much about the bible as many people here do, but I know enough about ancient and medieval history.