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.
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. 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