if (GBrowserIsCompatible()) {

      //copyright && copyright collection
      var copyOSM = new GCopyrightCollection('<a href="http://www.openstreetmap.org/">OpenStreetMap</a>');

      // apply the statement to the whole world
      copyOSM.addCopyright(new GCopyright(1, 
	  new GLatLngBounds(new GLatLng(-90, -180), new GLatLng(90, 180)), // region
	  0, // minimum zoom level
	  ' ' // no additional copyright message, but empty string hides entire copyright
      ));
  
      // create a tile layer
      var tilesMapnik = new GTileLayer(
	  copyOSM, // copyright statement
	  7, // minimum zoom level
	  18, // maximum zoom level
	  {
	      // options includes URL template for addressing individual tiles
	      // X, Y, Z respectively are tile coordinates for lattitude and longitude,
	      // and the zoom level
	      tileUrlTemplate: 'http://tile.openstreetmap.org/{Z}/{X}/{Y}.png',
	      isPng: true // otherwize assumes GIF images
	  }
      );

      //map type
      var mapMapnik = new GMapType(
	  [tilesMapnik], // list of layers
	  G_NORMAL_MAP.getProjection(), // borrow the Mercator projection from the standard map
	  'OSM' // name should be short enough to fit in button
      );
  

      var map = new GMap2(document.getElementById("map"));
      map.setCenter(new GLatLng(37, 0), 4);


      map.addMapType(mapMapnik);
      map.addControl(new GLargeMapControl());
      map.addControl(new GOverviewMapControl());
      map.addControl(new GScaleControl());

      map.setMapType(mapMapnik);

      map.enableDoubleClickZoom();
      map.enableContinuousZoom();
      map.enableScrollWheelZoom();



    }

custom gmap type

JavaScript