//API for Linear Gradient Fill
beginLinearGradientFill([0x000000,0xfffffff],[1,1],[0,255], startPoint:Point, endPoint:Point);

//API for Radial Gradient Fill
beginRadialGradientFill([0x000000,0xffffff],[1.0,1.0],[0,255], centerPoint:Point, radius:Number);

Below is snipets.

public function beginRadialGradientFill(	colors:Array,
	alphas:Array,
	ratios:Array,
	centerPoint:Point,
	radius:Number,
	spreadMethod:String="pad",
	interpolationMethod:String="rgb"):void{

	var mat:Matrix = new Matrix();
	mat.createGradientBox(radius*2,radius*2,0,centerPoint.x-radius,centerPoint.y-radius);
	graphics.beginGradientFill(GradientType.RADIAL, colors, alphas, ratios, mat, spreadMethod, interpolationMethod)
}

public function beginLinearGradientFill(	colors:Array,
	alphas:Array,
	ratios:Array,
	startPoint:Point,
	endPoint:Point,
	spreadMethod:String="pad",
	interpolationMethod:String="rgb"):void{
var dx:Number = endPoint.x-startPoint.x;
var dy:Number = endPoint.y-startPoint.y;
var dist:Number = Math.sqrt(dx*dx+dy*dy);
var rad:Number = Math.atan2(dy,dx);
var offsetX:Number = -(dist - Math.cos(rad)*dist)*0.5 + startPoint.x;
var offsetY:Number = -(dist - Math.sin(rad)*dist)*0.5 + startPoint.y;

var mat:Matrix = new Matrix();
mat.createGradientBox(dist,dist,rad,offsetX,offsetY);
graphics.beginGradientFill(GradientType.LINEAR, colors, alphas, ratios, mat, spreadMethod, interpolationMethod)
}