Then we know it's nowhere near 1,500 years old. The so-called Gospel of Barnabas can't be traced back earlier than about the 15th century. It's not ancient, it's medieval.
Not that a single copy of the "Bible" of dubious provenance is sufficent to overturn the entire Christian tradition anyway. Anyone opting for scholarship over sensationalism already realizes this.
Well, one would have to look at the text to see if it is the same Gospel of Barnabas-—could be a completely different text with the same name.