How to get Gmap winforms address in C#?

Question

I use Gmap in my project, and I want to show a address .

I tried to use BingMap, Yahoo, OpenStreet and googleMap. but I cant't show address. I wrote this code:

private void gMapControl1_Load(object sender, EventArgs e)
        {
            gmap.MapProvider = GMap.NET.MapProviders.GMapProviders.OpenStreetMap;
            GMap.NET.GMaps.Instance.Mode = GMap.NET.AccessMode.ServerOnly;
            gmap.Position = new GMap.NET.PointLatLng(20.992380, 105.788822);
            GMapOverlay markersOverlay = new GMapOverlay("markers");
            GMarkerGoogle marker = new GMarkerGoogle(new PointLatLng(), GMarkerGoogleType.red);
            marker.ToolTip = new GMap.NET.WindowsForms.ToolTips.GMapRoundedToolTip(marker);
            marker.ToolTipText = "Software Technologies";
            markersOverlay.Markers.Add(marker);
            gmap.Overlays.Add(markersOverlay);
        }

 private void gmap_OnMapClick(PointLatLng point, MouseEventArgs e)
        {
            var address = getAddressFromMap(point);
            rtbResultAddressGG.Text = "Address : " + String.Join(",", address.ToArray());
        }

private List<String> getAddressFromMap(PointLatLng point)
        {
            StringBuilder stringBuilder = new StringBuilder();
            List<Placemark> placemarks = new List<Placemark>();
            var sttCode = GMapProviders.GoogleMap.GetPlacemarks(point, out placemarks);
            if (sttCode == GeoCoderStatusCode.OK && placemarks != null)
            {
                List<String> address = new List<string>();
                foreach (var placemark in placemarks)
                {
                    address.Add(placemark.Address);
                }
                return address;
            }
            return null;
        }

I got the following error

GMap.NET.GeoCoderStatusCode.REQUEST_DENIED