In my WordPress lead automation app, I created a custom post type called Leads. When someone fills out the new lead Gravity Forms, it creates a new post in Leads. Each sales agent is an author and can view and edit their own leads. When using Gravity Forms to create a post type, you can only select a default author or assign the post to the logged in user. I needed to create a way to assign a lead to the sales agent assigned to manage it by the person creating the lead.
To do that I created a dynamic list of authors in my new lead form.
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. Add this to the functions file.
Function by Mike Madern
Then, you need to change the post author when the form is submitted.
In the code above, you need to change the 72 (in two places) to your form ID and 170 to the field ID that has the dropdown.