To set up a Geofence on Google Map, you need a Google Map object reference, then use the addCircle() function which expects CircleOptions() objects as a parameter. This is where you will input the center coordinates of the circle latitude, longitude, and radius of your geofencing. After completing a list of geofences, you can add it to a geofencing request.
Note that the Geofencing API here has charges per 1000 hits after consuming all the initial free requests. If you’ll pay for a Geofence feature alone, why not opt for a mobile app that has lots of features to comprehensively serve your purpose well.