Adding Google map layer in Openlayers - what about this function?

How to add Google map as a layer (something like "var gmap = new OpenLayers.Layer.Google("Google Streets", {visibility: false})"?) to this mymap.js function i already have:

var map = new OpenLayers.Map({
    div: 'map',
//  allOverlays: true,
    theme: null,
    projection: sphericalMercatorProj,
    displayProjection: geographicProj,
    units: 'm',
    numZoomLevels: 18,

    maxResolution: 156543.0339,
    maxExtent: new OpenLayers.Bounds(
        -20037508.34, -20037508.34, 20037508.34, 20037508.34
    ),
    controls: [
        new OpenLayers.Control.Attribution(),
        new OpenLayers.Control.Navigation(),
        new OpenLayers.Control.PanZoom(),
        new OpenLayers.Control.LayerSwitcher()


    ],
    layers: [
        new OpenLayers.Layer.OSM('OpenStreetMap', null),
        sundialsLayer,
        sprintersLayer,
        poisLayer
    ],
    center: new OpenLayers.LonLat(0, 0),
    zoom: 2
});

Thanks.

Note: suggested answer is fine -

    layers: [
        new OpenLayers.Layer.OSM('OpenStreetMap', null),
        new OpenLayers.Layer.Google(
        "Google Streets", // the default
        {numZoomLevels: 20},
        )


        sundialsLayer,
        sprintersLayer,
        poisLayer
    ],

Answer

In the layers array, just add the following:

new OpenLayers.Layer.Google(
                "Google Streets", // the default
                {numZoomLevels: 20}
            )

In addition, you need to make sure that you refer to the Google Maps Library, by using the following tag:

For a complete sample, have a look at: Google (v3) Layer Example