How I can give a particular style to maps in drupal site based on gmap module
I managed to do it by changing a little bit the gmap.js file. At the line 358

obj.bind("boot", function () {
    obj.map = new google.maps.Map(elem, obj.opts);
});


Add these line:

obj.bind("boot", function () {
     obj.opts.styles = [
                                        {
                                            "featureType": "water",
                                            "stylers": [
                                                { "color": "#808084" }
                                            ]
                                        },{
                                            "featureType": "road",
                                            "stylers": [
                                                { "color": "#c8cacc" }
                                            ]
                                        },{
                                            "featureType": "landscape.man_made",
                                            "stylers": [
                                                { "color": "#b0b3b4" }
                                            ]
                                        },{
                                            "featureType": "road",
                                            "elementType": "labels.text.fill",
                                            "stylers": [
                                                { "color": "#000000" }
                                            ]
                                        },{
                                            "featureType": "poi",
                                            "stylers": [
                                                { "color": "#9c9ea2" }
                                            ]
                                        },{
                                            "featureType": "poi",
                                            "elementType": "labels.text.fill",
                                            "stylers": [
                                                { "color": "#000000" }
                                            ]
                                        },{
                                            "featureType": "administrative",
                                            "elementType": "labels.text.fill",
                                            "stylers": [
                                                { "color": "#808080" }
                                            ]
                                        },{
                                            "featureType": "landscape.natural",
                                            "stylers": [
                                                { "color": "#808084" }
                                            ]
                                        },{
                                            "featureType": "transit",
                                            "stylers": [
                                                { "color": "#c9c9cc" }
                                            ]
                                        },{
                                            "featureType": "transit.station.rail",
                                            "stylers": [
                                                { "color": "#838080" }
                                            ]
                                        },{
                                            "featureType": "road.arterial",
                                            "stylers": [
                                                { "color": "#adaeb1" }
                                            ]
                                        },{
                                            "featureType": "poi",
                                            "elementType": "labels.icon",
                                            "stylers": [
                                                { "color": "#3e3d3a" },
                                                { "visibility": "off" }
                                            ]
                                        },{
                                            "featureType": "transit.station",
                                            "elementType": "labels",
                                            "stylers": [
                                                { "visibility": "off" }
                                            ]
                                        },{
                                        }
                                    ];
    obj.map = new google.maps.Map(elem, obj.opts);

});


Also check this thread:

drupal.org