In your GME code, you can refer to most of the popular custom namespaces (such as gd, digg, itms) directly. GME will assign an internal alphanumeric ID (e.g. "ns1432138428") for namespaces it doesn't recognize. To discover these namespace IDs, use the built-in Feed Browser:
- Click on the Feed Browser tab
- Select Remote Feed from the pull-down menu
- Enter the URL of your feed