If you add a Google Account as reviewer that does not have Mail 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
Step 1 reviewer sends Request for approval email to Step 2 reviewer
Step 2 reviewer sends Request for approval email to Step 3 reviewer, etc..
If the reviewer email account does have Email enabled and the Request ID still does not move onto the next Step reviewer, please contact us at email@example.com indicating the Request ID number.