Ok, so after a few more tests... i've once again used Gravity forms to create a 'draft' post on my WP site.
Then i checked the DB and noticed that the post_name field isn't actually filled in at this stage.
So when i then went back and hit the default WP 'Publish' button it filled in the field as it should with the 'slug'.
So, the script i mentioned above isn't technically doing anything wrong as i initially thought.
So, my next question is... is it even possible to manually change the 'post_status' to 'publish' via a MySQL command or other method and actually have it fill in the post_name filed (and also the post_date_gmt field - which i also noticed is left blank at draft status)??