Edit existing post from gravity form, without seeing WordPress back-end

This isn't something Gravity Forms does by default. Gravity Forms is designed to create data from the front end, but Entries can only be edited via the backend *AND* when Gravity Forms creates a post it simply creates the Post and it's up to using the standard WordPress tools to manage the post after it's created.

It's possible to make Gravity Forms edit existing posts/pages, but it requires writing custom code and using available API hooks to tell Gravity Forms to do this. It's something you would need quit a bit of Gravity Forms customization experience in order to implement it.

That being said, you could probably use another plugin for the post editing. There are a few plugins in the WordPress.org repository that let you do things like edit posts/pages on the front end if the user logged in is the post author. I would look there.

Finding a way to allow users to use Gravity Forms to edit existing data as well as create data is something we would like to add as a feature in the future once we determine an intuitive and easy to use way to implement this so users understand how to use it properly.