The wikipedia entry for Christian Metal says that some of the stations which do play said genre:
as a whole these larger stations generally maintain a GMA-prescribed format.
That would seem to exclude metal since the GMA's mission statement apparently is:
Founded in 1964, the Gospel Music Association serves as the face and voice for the Christian/Gospel music community and is dedicated to exposing, promoting and celebrating the gospel through music of all styles including pop, rock, praise & worship, black gospel, R&B;, hip hop, southern gospel, country, and more.
I guess it says all styles "including" and not to the exclusion of. But what is the connection between GMA and the many Contemporary Christian Music stations around the country and what, exactly, would be likely to not meet their seal of quality?