How to add gradient layer to GMSMapView

Question

In my project I have TableView which consist GMSMapView . I want to show gradient to GMSMapView . I have tried following code

class Colors {
    let colorTop = UIColor(red: 192.0/255.0, green: 38.0/255.0, blue: 42.0/255.0, alpha: 1.0).CGColor


    let colorBottom = UIColor(red: 35.0/255.0, green: 2.0/255.0, blue: 2.0/255.0, alpha: 1.0).CGColor


    let gl: CAGradientLayer

    init() {
        gl = CAGradientLayer()
        gl.colors = [ colorTop, colorBottom]
        gl.locations = [ 0.0, 1.0]
    }
}

And this Code in TableView Cell's Class

class AllTripTableViewCell: UITableViewCell {
    var colors = Colors()

    func refresh() {
        mapView.backgroundColor = UIColor.clearColor()
        let backgroundLayer = colors.gl
        backgroundLayer.frame = mapView.frame
        mapView.layer.insertSublayer(backgroundLayer, atIndex: 0)
    }


}

and use like this

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell
     {
        let cell = tableView.dequeueReusableCellWithIdentifier("AllTrip", forIndexPath: indexPath) as! AllTripTableViewCell

cell.refresh()
     }

But It doest Show gradient Layer