edit form code and database fields

You wouldn't add your own JS code. You would use the built in conditional logic. The submit button supports conditional logic so you can show/hide the submit button when a user answers a specific question. Conditional logic only works on drop down, multiple choice or checkbox selections. It's found under Form Settings. You would use that to show/hide the submit button.

I'm not sure what you are referring to as far as Posts table...

Gravity Forms by default stores data in it's own database tables, not in the WordPress posts table. These are called Entries and forms always create an Entry.

To create WordPress Posts you must use the Post Fields. ONLY POST FIELDS CREATE WORDPRESS POSTS. If you aren't using Post Fields, that data isn't stored in the Posts table.

The Post Fields consist of Post Title, Post Body, Post Category, Post Tags, Post Image and Custom Field. These are the only fields that create data in the WordPress posts table. So if you want to create a Post you need to use these fields.

If you want to insert the value of fields you have added into your Post Body content you can do this using the Content Template functionality of the Post Body field. The Content Template functions similar to the Notification emails which allow you to insert form field values into the content and they are replaced with the actual field values. The Content Template functionality is available when you edit a Post Body field.