This is a device setting, so the exact steps may be different based on your device. For further reference you may need to seek support from your device or operating system manufacturer.
Below are the steps for making sure that maps are meant to open in the app on an iPhone. First you must get to the google apps settings. This is available through the Gmail app.
Head to the settings in your gmail app
Next, head to your google apps settings.
Then in the "Open Maps in" section, select "Google Maps"
If after checking this setting the maps are still opening in the browser you will need to contact support from either google, your operating system, or the manufacturer of your device.