There must be a Google account associated with an email address for it to be accepted by Google Apps Script as a reviewer/approver.
Create your free google account here: http://goo.gl/wbP8g1
A non-Google account cannot be editor in Apps Script. The account owner will only get a notification and then a pop-up will ask him to associate his email to a Google account.
If you try to add a non-Google email address as reviewer to your Form Workflow Plus, you will get an error message.
If you add a Google Account as reviewer that does not have Gmail enabled the reviewer will be able to approve a Request on the Dashboard but the Request ID will not move onto the next level reviewer.
If you have selected an Email Address on the Advanced tab the Requester will not receive the reviewers decision once acted on if Gmail is not enabled.
The workflow owner email address will send the Request for approval email to Step 1 reviewer. This email is sent from the add-on using MailApp.
Step 1 reviewer sends Request for approval email to Step 2 reviewer.
Step 1 reviewer will also send an email notification to the Requester if Email Address is selected on the Advanced tab. This email is sent using GmailApp
Step 2 reviewer sends Request for approval email to Step 3 reviewer, etc..This email is sent using GmailApp etc