Leaflet plugin that enables the use of Google Map tiles - http://matchingnotes.com/using-google-map-tiles-with-leaflet

leaflet-google.js

/*
 * L.TileLayer is used for standard xyz-numbered tile layers.
 */
L.Google = L.Class.extend({
	includes: L.Mixin.Events,

	options: {
		minZoom: 0,
		maxZoom: 18,
		tileSize: 256,
		subdomains: 'abc',
		errorTileUrl: '',
		attribution: '',
		opacity: 1,
		continuousWorld: false,
		noWrap: false,
	},

	// Possible types: SATELLITE, ROADMAP, HYBRID
	initialize: function(type, options) {
		L.Util.setOptions(this, options);

		this._type = google.maps.MapTypeId[type || 'SATELLITE'];
	},

	onAdd: function(map, insertAtTheBottom) {
		this._map = map;
		this._insertAtTheBottom = insertAtTheBottom;

		// create a container div for tiles
		this._initContainer();
		this._initMapObject();

		// set up events
		map.on('viewreset', this._resetCallback, this);

		this._limitedUpdate = L.Util.limitExecByInterval(this._update, 150, this);
		map.on('move', this._update, this);
		//map.on('moveend', this._update, this);

		this._reset();
		this._update();
	},

	onRemove: function(map) {
		this._map._container.removeChild(this._container);
		//this._container = null;

		this._map.off('viewreset', this._resetCallback, this);

		this._map.off('move', this._update, this);
		//this._map.off('moveend', this._update, this);
	},

	getAttribution: function() {
		return this.options.attribution;
	},

	setOpacity: function(opacity) {
		this.options.opacity = opacity;
		if (opacity < 1) {
			L.DomUtil.setOpacity(this._container, opacity);
		}
	},

	_initContainer: function() {
		var tilePane = this._map._container
			first = tilePane.firstChild;

		if (!this._container) {
			this._container = L.DomUtil.create('div', 'leaflet-google-layer leaflet-top leaflet-left');
			this._container.id = "_GMapContainer";
		}

		if (true) {
			tilePane.insertBefore(this._container, first);

			this.setOpacity(this.options.opacity);
			var size = this._map.getSize();
			this._container.style.width = size.x + 'px';
			this._container.style.height = size.y + 'px';
		}
	},

	_initMapObject: function() {
		this._google_center = new google.maps.LatLng(0, 0);
		var map = new google.maps.Map(this._container, {
		    center: this._google_center,
		    zoom: 0,
		    mapTypeId: this._type,
		    disableDefaultUI: true,
		    keyboardShortcuts: false,
		    draggable: false,
		    disableDoubleClickZoom: true,
		    scrollwheel: false,
		    streetViewControl: false
		});

		var _this = this;
		this._reposition = google.maps.event.addListenerOnce(map, "center_changed", 
			function() { _this.onReposition(); });
	
		map.backgroundColor = '#ff0000';
		this._google = map;
	},

	_resetCallback: function(e) {
		this._reset(e.hard);
	},

	_reset: function(clearOldContainer) {
		this._initContainer();
	},

	_update: function() {
		this._resize();

		var bounds = this._map.getBounds();
		var ne = bounds.getNorthEast();
		var sw = bounds.getSouthWest();
		var google_bounds = new google.maps.LatLngBounds(
			new google.maps.LatLng(sw.lat, sw.lng),
			new google.maps.LatLng(ne.lat, ne.lng)
		);
		var center = this._map.getCenter();
		var _center = new google.maps.LatLng(center.lat, center.lng);

		this._google.setCenter(_center);
		this._google.setZoom(this._map.getZoom());
		//this._google.fitBounds(google_bounds);
	},

	_resize: function() {
		var size = this._map.getSize();
		if (this._container.style.width == size.x &&
		    this._container.style.height == size.y)
			return;
		this._container.style.width = size.x + 'px';
		this._container.style.height = size.y + 'px';
		google.maps.event.trigger(this._google, "resize");
	},

	onReposition: function() {
		//google.maps.event.trigger(this._google, "resize");
	}
});

Licence for this script is really minor issue compared to debatable status of whole plugin - is it legal to use Google maps this way or not :)

@shramov unfortunately those are two different questions, which are both important.

• As we are using Free Software in a commercial setting, the current Leaflet.GridLayer.GoogleMutant will not pass any serious licensing clearing house (precisely there is no permission to use it, because the license of you and Bergot is violated).
• When using the Google Maps API with a key, a number of people evaluated this as acceptable. But this is a matter between different parties: google and the organisation that bought the api key from them. Maybe this should or is discussed in more depth somewhere else?

Is there a final agreement on whether using the google maps API tiles without showing/using the shell is legal?