Go to the “Posts->All”. The 2nd column in a view is “Author”. Find a post, created via Gravity Forms, which you wish allow to edit to some user. Are all created from the same user? Point by mouse to the user name at the “Author” column. You will see the user ID at the link, like 20 here: wp-admin/edit.php?post_type=post&author=20

Then you can use “edit restrictions” add-on and input to your user profile the author ID:
section “Posts/Pages/Custom Post Types Editor Restrictions”, field “with author user ID (comma separated)”.