I have CiviCRM and Gravity form both installed on my Wordpress website. I also bought the CiviCRM + Gravity Form plugin that integrate them together. I want to see how will I be able to use CiviCRM database to autofill the contact information after he logs into the website. For example I have a user who wants to fill out this application, and he already has a WP username and password. So when he logs in, I want this form to autofill his contact information (prefix, FirstName, LastName, Email, phone number, and the full address). Obviously, it should be getting this information from his CiviCRM database.
If you paid for Gravity Form-CiviCRM integration, I would contact the authors for support.
Meanwhile, most of the community uses Caldera Form with Caldera Form-CiviCRM integration. I know this feature exists with Caldera Form-CiviCRM but haven't used Gravity Form integration.
To add, I had used the Gravity Forms integration quite a bit as of late 2017 and even if you get it to auto-fill, it will not update an existing record in CiviCRM. It's just not nearly as developed as Caldera Forms integration and would consider it stuck in the development phase.
So... you should just make it easy on yourself and use Caldera Forms.