The following code, when inserted into a custom plugin or your child theme's functions.php file, will automatically delete the associated WordPress "Users" account when deleting a User from the "Trash" tab.

gravity-forms-auto-delete-wp-user.php

<?php

// GF Auto Delete User from WP Upon Form Entry Deletion
// If user entry (in members form) is trashed, then deleted from trash, they are also deleted from WP Users table.
// https://endurtech.com/delete-associated-wp-user-when-deleting-gravity-forms-entry/

add_action( 'gform_delete_entry', function ( $entry_id )
{
  if ( ! function_exists( 'gf_user_registration' ) )
  {
    return;
  }
  $user_id = gf_user_registration()->get_user_by_entry_id( $entry_id, true );
  wp_delete_user( $user_id );
});

?>