Go to 3.2. In command line write "V" and give the GDG base name at data set name position. Press enter. then Give process request as 2 (Delete) and Data type as 4 (GDG). Press enter. One JCL will come. Write EXEC in command line and press enter. It will delete the GDG base.
You can use following:
DELETE <GDG base name> -