public IList<MapPoint> GetClusters(YourFilterObj filter)
{
    var clusterPointsCacheKey = "somecachekey";
    var points = GetClusterPointCollection(clusterPointsCacheKey);

    var mapService = new ClusterService(points);
    var input = new GetMarkersParams()
    {
        NorthEastLatitude = filter.NorthEastLatitude,
        NorthEastLongitude = filter.NorthEastLongitude,
        SouthWestLatitude = filter.SouthWestLatitude,
        SouthWestLongitude = filter.SouthWestLongitude,
        ZoomLevel = filter.ZoomLevel,
        PointType = clusterPointsCacheKey
    };

    var markers = mapService.GetClusterMarkers(input);

    return markers.Markers;
}

private PointCollection GetClusterPointCollection(string clusterPointsCacheKey)
{
    var points = new PointCollection();
    if (points.Exists(clusterPointsCacheKey))
        return points;

    var dbPoints = GetPoints(); // Get your points here
    var mapPoints = dbPoints.Select(p => new MapPoint() { X = p.X, Y = p.Y }).ToList();
    var cacheDuration = TimeSpan.FromHours(6);
    points.Set(mapPoints, cacheDuration, clusterPointsCacheKey);

    return points;
}

public class YourFilterObj
    {
        public double NorthEastLatitude { get; set; }
        public double NorthEastLongitude { get; set; }
        public double SouthWestLatitude { get; set; }
        public double SouthWestLongitude { get; set; }
        public int ZoomLevel { get; set; }
    }

private List<MapPoint> GetPoints()
        {
            List<MapPoint> mapPoint = new List<MapPoint>();

            using (SqlConnection cs = new SqlConnection(Properties.Settings.Default.DatabaseObjectConnectionString))
            {                
                SqlDataAdapter da = new SqlDataAdapter("SELECT GeoLatitude, GeoLongitude FROM TableAddressObj", cs);
                DataTable dt = new DataTable();
                da.Fill(dt);

                foreach (DataRow row in dt.Rows)
                {
                    try
                    {                        
                        mapPoint.Add(new MapPoint(Convert.ToDouble(row[0]), Convert.ToDouble(row[1])));
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message, "Error");
                    }
                }
            }
            return mapPoint;            
        }

private void Button1_Click(object sender, EventArgs e)
        {
            gMapControl.Overlays.Clear();

            YourFilterObj region = new YourFilterObj()
            {
                NorthEastLatitude = gMapControl.FromLocalToLatLng(0, 0).Lat,
                NorthEastLongitude = gMapControl.FromLocalToLatLng(0, 0).Lng,
                SouthWestLatitude = gMapControl.FromLocalToLatLng(gMapControl.Width, gMapControl.Height).Lat,
                SouthWestLongitude = gMapControl.FromLocalToLatLng(gMapControl.Width, gMapControl.Height).Lng,
                ZoomLevel = 7
            };

            Bitmap LogoMarker = (Bitmap)Image.FromFile("Image\\logo64x64.png");
            // Оголошуємо змінну список маркерів
            GMapOverlay markers = new GMapOverlay("markers");
            // 1. Створюємо на контролі список маркерів (щоб маркери відображались коректно потрібно створити список, а потім накидати туди маркерів)
            gMapControl.Overlays.Add(markers);

            IList<MapPoint> mapPoint = GetClusters(region);

            foreach (MapPoint point in mapPoint)
            {
                GMapMarker markerObject = new GMarkerGoogle(new PointLatLng(point.Lat, point.Long), LogoMarker)
                {
                    ToolTipText = "перевірка",
                    Tag = "для тесту",
                    ToolTipMode = MarkerTooltipMode.OnMouseOver
                };

                markers.Markers.Add(markerObject);
            }

            gMapControl.Refresh();
        }