1. Insert that object on the plan (prefferable at 0,0,0 coordinates)
2. Select it; menu File / Libraries and Objects / Save Selection As... /
Name: (type in the name), Type: (.gsm), Save to: (Embedded Library) /
Save
3. Now you can modify the new created object: select it, File / Libraries and Objects / Open Object...Welcome to the GDL world.
4. Make the modifications, then Save.
5. You can export that object in a folder of your choice from the Library Manager, to use it in other projects