Places uses the native iOS and Google Play location services to determine user location. They combine GPS, cell tower, and WiFi location for maximum accuracy with minimum battery drain.
Users with GPS or WiFi disabled can still trigger Places as long as background Location Services are enabled for the app, but smaller geofences ( < 1km) might not work properly, since cell tower location data is less accurate.
To get a better background on Places & geofences read up on it here as well as more information on Messaging with Places.