You can add polygons in GMaps.js this way:
path = [[-12.040397656836609,-77.03373871559225], [-12.040248585302038,-77.03993927003302], [-12.050047116528843,-77.02448169303511], [-12.044804866577001,-77.02154422636042]];
map.drawPolygon({
paths: path,
strokeColor: '#131540',
strokeOpacity: 0.6,
strokeWeight: 6
});
The path of the polygon is defined by an array of array of two (latitude and longitude).
NOTE: Also, you can add a GeoJSON Polygon or MultiPolygon path using useGeoJSON: true.