gmaps.js

You can attach a geofence (which can be a polygon or a bounds) to a marker with:

polygon = map.drawPolygon({
  paths: path,
  strokeColor: '#BBD8E9',
  strokeOpacity: 1,
  strokeWeight: 3,
  fillColor: '#BBD8E9',
  fillOpacity: 0.6
});
map.addMarker({
  lat: -12.043333,
  lng: -77.028333,
  draggable: true,
  fences: [polygon],
  outside: function(marker, fence) {
    alert('This marker has been moved outside of its fence');
  }
});

You must define an outside callback, which will use this marker and its fence.

NoteYou also can use checkMarkerGeofence or checkGeofence methods.