func mapView(mapView: MKMapView, rendererForOverlay overlay: MKOverlay) -> MKOverlayRenderer {
    if let overlay = overlay as? MKPolyline {
            /// define a list of colors you want in your gradient
            let gradientColors = [UIColor.green, UIColor.blue, UIColor.yellow, UIColor.red]

            /// Initialise a GradientPathRenderer with the colors
            let polylineRenderer = GradientPathRenderer(polyline: overlay, colors: gradientColors)

            /// set a linewidth
            polylineRenderer.lineWidth = 7
            return polylineRenderer
    }
}

polylineRenderer.border = true
polylineRenderer.borderColor = UIColor.red /* defaults to white if not specified*/