What is the GJXDM or NIEM namespace?
The namespace is used for schema version control, in that the contents of GJXDM or NIEM (elements, types, attributes, and attribute groups, as well as the definitions of all of these things) will never change for a given namespace identifier. That is, for example, the contents of the GJXDM namespace at version 3.0.3 will always be the same for that namespace. And any changes to the data model will require a new namespace identifier.
Example of a namespace: http://www.it.ojp.gov/jxdm/3.0.3/
Directory of all GJXDM namespace releases: http://www.it.ojp.gov/jxdm
NIEM namespaces: http://www.niem.gov/niem/namespaces