On gmail (on pc), you can set up very flexible rules (gmail calls them "filters") which take various actions based on incoming message title, sender, content etc. One rule you can set up is to notify your phone via sms of certain gmail (that meets you criteria). Once you are notified by sms, results depend on your sns app.... my sms app (S4 stock app) has setting for recurring notification every 2 minutes until I view the sms.