Send Notification to all in a Category

Is this possible? Sure. But it isn't something that can be done easily.

It's easy to send the admin notification to an email address associated with a custom field for an individual page or post that the form is displayed on. As you know, that can be done simply by using merge tags and the WordPress custom field functionality.

However, doing the same thing with multiple posts that are displayed on a category page? Much more complex.

You can send the admin notification email to whoever you want using the gform_notification_email hook to define where the notification should be sent. That hook is documented here:

http://www.gravityhelp.com/documentation/page/Gform_notification_email

The complexity involved is you would have to write custom PHP that can get ALL of those custom field emails across all of the posts appearing on that category page and then combine those values into a single string that you then pass to the gform_notification_email hook.

It is even more complex if paging is involved on your category page. Is it supposed to get all of them or just the ones that appear on that page?

So it is certainly possible. You can tell Gravity Forms to send the email notification to wherever you want using PHP. But you need to be able to know how to get those emails from wherever they are stored in order to do so.