In my WordPress lead automation app, when a sales person submits a deal, the processor selects their name from a dropdown in order to start processing the deal. In order to avoid having to add a new name to the Gravity Forms dropdown field every time we get a new sales person, I repurposed this Gravity Forms code to dynamically populate a dropdown field with a list of all active users who are sales people and have the user role author.
To use the code for the field, add populate-posts to the custom CSS class field in the field you want to dynamically populate.
If you just wanted to populate a list of authors or other role in a dropdown, then that’s it. To actually assign the post or custom post type that you are creating, you need to convert the user display name to their user ID and then change the post author when submitting the form. Here’s a tutorial for that step.