Gravity Forms `form_submit` event

## Feature Description

There were `form_submit` events added for several form plugins noted in #1728, but not Gravity Forms. Assuming the [event-providers](https://github.com/google/site-kit-wp/tree/develop/assets/js/event-providers) is an indicator to confirm, Gravity Forms is also not listed there.

Note that I have not tested the plugin for whether this does somehow otherwise track the event.

---------------

_Do not alter or remove anything below. The following sections will be managed by moderators only._

## Acceptance criteria

* `form_submit`pushed after submitting form created with Gravity Form

## Implementation Brief

* [ ] Ajax `gform_confirmation_loaded` (see https://docs.gravityforms.com/gform_confirmation_loaded/)
* [ ] Not ajax `gform_after_submission` (see https://docs.gravityforms.com/gform_after_submission/)

### Test Coverage

* N/A?

## QA Brief

* N/A?

## Changelog entry

* Add `form_submit` event for Gravity Forms

Start by reviewing the existing providers under assets/js/event-providers and the related implementation from issue #1728. Verify the Gravity Forms Ajax gform_confirmation_loaded and non-Ajax gform_after_submission hooks, then ensure form_submit is pushed after submission and validate both paths with the available project tests or QA steps.

Written by the indexing model from the issue text.