WPTrack is a pixel tracking plugin for wordpress that allows you to create a tracking pixel that you may insert into any webpage. Views are then tracked and timestamped along with the ip address of the user who views the tracking pixel.
This plugin also contains gravity forms integration so that you may enable it for form notifications. This is enabled under the gravity form's settings, in it's own menu called "WP Track."
To use this, you need to enable wp-track on the form, and check the box for any notifications.
Before the notification is sent out, a tracking pixel is created and embedded into the notification. You can check how many times the notification email has been viewed by navigating to the form entry and look for the meta box named "WP Track" where you will find a list of views.