I disagree here. The gospels are evidence and the JK Rowling books are kind of evidence too.
The issue is how reliable are they? The Rowling books are explicitly stated by the author to be unreliable evidence (i.e., fiction) for the existance of Hogwarts.
The gospels might therefore be taken as a tich more "reliable" than the Harry Potter series.
But, as you suggest (but could be clearer about), it takes other sources of evidence to support a written work as reliable.
In addition, other than some minor things (e.g., the warning about the ides of march), the written material from Roman times is not flooded with magic and the supernatural which is part of the evidence against
the reliability of the gospels.
So I agree with GDR, the NT is evidence. However, it is a demonstration of how weak his case is because it is such obviously poor evidence.