This members-only tutorial provides the steps to have the post-specific file to begin downloading after a Gravity Forms form has been filled and submitted. We shall use Meta Box to set up post-specific File Input field and have the form inside Oxygen's Modal.
Change the Default Confirmation type to Redirect and set any redirect URL (could be your site's homepage or even https://google.com - it doesn't matter).
Here is an export of the form from my test site for your reference. Mirror.
Step 2 - CPT and Custom Field
In my test site I registered a CPT called download using Meta Box AIO, then a field group attached to it having a File Input type of field with the ID of file_input_i4wa4evst5. This is for setting post-specific file that the visitors can download.
You can of course implement this tutorial for any post type incl. the built-in post and/or page.
Also, a different plugin like ACF can be used for registering the custom field.
Step 3 - Populate the Field
Edit your posts (or CPT entries) and select file for each.