File - GDL Objects - New Object. This will bring up the GDL scripting screen. Click the "Select Subtype" button and in the subtype tree, expand the "Drawing Symbols" and "Marker" headings to see the various marker types available. Select the style you want and then press select.
This process will bring the list of default parameters into your object and will also let the program recognize the object as the type desired. From there, write your code for the symbol under the 2D Script and you should be good to go.
Good luck
