Forward Google Mail (Gmail) emails to a SMS
You can't use email-to-sms to forward gmail emails as an SMS because the emails are sent from 'email@example.com' which is a generic email address used for all Gmail accounts. Since we use the from address as authentication we won't know how to differentiate different clients.
We do have a few solutions though:
Zapier is a great way to link multiple services together. Depending on your usage you can sign up for a free or paid account.
Instructions: Use Zapier to link Gmail and ClickSend
b) Dedicated Forwarding Address.
You can setup an email address to forward to. More info here.