Not tried this app but a brief search for Recurring Notifications on the play store brought this up at the top of the list:
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.
I use LightFlow to handle all notifications on my phone. It allows for repeating alerts for just about every application (gmail included). It has a ton of great features like setting different color LEDs for different notifications. For instance, I have gmail blink green, SMS blink magenta, missed calls blue, and so on.
There is also a Lite version that doesn't support as many apps, but gmail is supported. I recommend trying out the free version and setting it up the way you want. Then you can backup your settings before buying the full version.
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