You can also arrive at this dialog by clicking on the "Custom" button in the Waypoints=>Edit dialog
The grid shows all your current custom data fields. You can add, edit, and delete custom fields from here
Note: GSAK has reserved field names which will automatically get updated internally by GSAK. Currently the only such reserved field name is the string field "GPXFile" (without the quotes) which will automatically get updated with the "name" of the GPX file which is loaded into GSAK. For more information see this link