You also need to appreciate that a form’s inputs are stored as entries in your database, separately to any merged document that may be created out of them. As such, selecting the ‘Delete Document’ option in [Form] > Settings > GravityMerge will delete the merged document, but the form entry will still exist.
If you wish to delete the form entries as well, you can set a data retention policy for your form in the [Form] > Settings > Personal Data area. In the example below, entries will be retained for 2 days and will then be deleted permanently, automatically.
For more information on these settings, see the Gravity Forms documentation at https://docs.gravityforms.com/personal-data-settings/