Submitting images to gallery

Gravity Forms only creates new posts. It does not have built in options for updating an existing post.

It is possible to use it to update an existing post instead of creating a new one, but only through the use of available developer hooks and filters to customize Gravity Forms default behavior.

You would need to implement a customization using code that tells Gravity Forms not to create a new post, but instead to add the Post Image that is uploaded to an existing post and which post that would be using it's id.

Plenty of people have customized Gravity Forms to update an existing post rather than create a new one, however i'm not aware of any existing tutorials that anyone has written that explains step by step how to do it.

Here is another support forum post that discusses the same thing:

http://www.gravityhelp.com/forums/topic/users-upload-images-to-existing-post

In that post I provided a link to a tutorial that explains how to use Gravity Forms to update an existing users profile rather than create a new user. The concepts used are going to be the same so you could use that tutorial as a reference to put together the custom code to accomplish what you want to do.