To access SMS Notifications, navigate to Forms in the left-hand menu, then Settings. Click on Twilio.
Twilio provides a web-service API for businesses to build scalable and reliable communication apps. Sign up for a Twilio account to receive SMS messages when a Gravity Form is submitted.
Once you have signed up for Twilio, come back to your dashboard > Forms > Settings > Twilio and add in the Account info here to finalize setup.