You can add a GeoJSON Polygon or MultiPolygon in GMaps.js in the following way:
polygon = map.drawPolygon({
paths: paths,
useGeoJSON: true,
strokeOpacity: 1,
strokeWeight: 3,
fillColor: '#BBD8E9',
fillOpacity: 0.6
});
NOTE: The GeoJSON Polygon should be surrounded by brackets, according the specification.