Hello,
If you want to delete all of the current generations, you can use this:
//DELETE EXEC PGM=IEFBR14
//D1 DSN=THEGDG.DSN,DISP=(OLD,DELETE,KEEP)
All cataloged generations will be deleted and the base will remain available for use when new files are cataloged.
Suggest you create a "dummy" gdg base, create a few entries, and test the delete if this is something you have not seen/used before.