Calendars allows to open a map when a Location is assigned to an event. To do that, tap the event and then tap the location link.
To change the map you want to launch, when tapping a location on the event, follow the steps below:
- Go to Calendars Settings.
- Tap View Options.
- Scroll down and choose Google or iOS maps.
After that, once you tap on the link, the appropriate map app will be launched.
Keep in mind that you need to have the Google Maps app installed to be able to select it in the settings.