GraphicsGradient.radial(num startX, num startY, num startRadius, num endX, num endY, num endRadius)

Source 

GraphicsGradient.radial(this.startX, this.startY, this.startRadius, this.endX, this.endY, this.endRadius) :
    this.kind = "radial",
    this.colorStops = new List<GraphicsGradientColorStop>();