GradientView.cs

// from https://forums.xamarin.com/discussion/comment/279890/#Comment_279890

public partial class GradientView : ContentView
{
    public Color StartColor { get; set; } = Color.Transparent;
    public Color EndColor { get; set; } = Color.Transparent;
    public bool Horizontal { get; set; } = false;

    public GradientView()
    {
        InitializeComponent();

        SKCanvasView canvasView = new SKCanvasView();
        canvasView.PaintSurface += OnCanvasViewPaintSurface;
        Content = canvasView;
    }

    void OnCanvasViewPaintSurface(object sender, SKPaintSurfaceEventArgs args)
    {
        SKImageInfo info = args.Info;
        SKSurface surface = args.Surface;
        SKCanvas canvas = surface.Canvas;

        canvas.Clear();

        var colors = new SKColor[] { StartColor.ToSKColor(), EndColor.ToSKColor()};
        SKPoint startPoint = new SKPoint(0,0);
        SKPoint endPoint = Horizontal ? new SKPoint(info.Width, 0) : new SKPoint(0, info.Height);

        var shader = SKShader.CreateLinearGradient(startPoint, endPoint, colors, null, SKShaderTileMode.Clamp);

        SKPaint paint = new SKPaint
        {
            Style = SKPaintStyle.Fill,
            Shader = shader
        };

        canvas.DrawRect(new SKRect(0, 0, info.Width, info.Height), paint);
    }
}