Apple - To change default map application in iPhone

You can't change the default app. Some apps give you the option to choose which app to open maps/directions in, but this varies per app and app developer. There is no system toggle and Safari/Siri/etc will continue to open maps in Maps.

As of iOS 10 you can delete the iPhone maps application. If you click on a Google Calendar destination, you will then get a message saying the Maps app is missing. Ignore that message. Hold down a finger on the address, select copy, open Google Maps and paste the address you just copied. This saves a few key strokes and it works.