In order to create a Gravity subscription form, you'll need to be logged into both your WordPress Site and FeedBlitz to complete this action.
Go to the Form in Gravity Forms in your WP account. Edit the form.
Look for Send to Email:
In this field you will place an email generated from your FeedBlitz List.
Log into FeedBlitz and select your newsletter. Look for Parsers in the grey tile labeled Subscriber Management.
Name the parser something that makes sense to you.
Set the Trigger when: Subject contains [insert what you set the subject of your Gravity form to be]
Email Forwarding: Always (this enables you to ensure it is working and can be changed once you are sure it is working)
The next part is controlled by the custom fields associated with this list. If you only have email, that is the only information that will be assigned. Collecting more via Gravity Forms won't add it to the list, you can add more custom fields using Publisher Tools in the left hand menu of FeedBlitz.
Once you have this done click Create Parser.
This will generate a long email address, copy this to your Gravity Form Notifications, email sent to field. Click Update Notification.
Test to make sure your parser is working by subscribing with an email address. You should get a notification saying Match or Mismatch.
The parser will only add people who are new to the list, so to fully test, be sure to valid, but not already subscribing email address.
Please let me know if you have any further questions. More information on Parsers can be found in Making the Most of FeedBlitz. http://assets.feedblitz.com/MakingTheMostOfFeedBlitz.pdf