An ExtensionHandler for Apache Batik's svggen part that uses SVGGraphics2D to convert Swing drawing routines to SVG elements. Currently supports LinearGradientPaint and RadialGradientPaint. Feel free to comment and extend ..

GradientExtensionHandler.java

import static org.apache.batik.util.SVGConstants.*;

import java.awt.Color;
import java.awt.LinearGradientPaint;
import java.awt.MultipleGradientPaint;
import java.awt.Paint;
import java.awt.RadialGradientPaint;
import java.awt.geom.AffineTransform;
import java.awt.geom.Point2D;

import org.apache.batik.svggen.DefaultExtensionHandler;
import org.apache.batik.svggen.SVGColor;
import org.apache.batik.svggen.SVGGeneratorContext;
import org.apache.batik.svggen.SVGPaintDescriptor;
import org.w3c.dom.Element;

/**
 * Extension of Batik's {@link DefaultExtensionHandler} which 
 * handles different kinds of Paint objects
 * 
 * I wonder why this is not part of the svggen library.
 * @author Martin Steiger
 */
public class GradientExtensionHandler extends DefaultExtensionHandler
{
  @Override
	public SVGPaintDescriptor handlePaint(Paint paint, SVGGeneratorContext genCtx)
	{
		// Handle LinearGradientPaint
		if (paint instanceof LinearGradientPaint)
			return getLgpDescriptor((LinearGradientPaint) paint, genCtx);
			
		// Handle RadialGradientPaint
		if (paint instanceof RadialGradientPaint)
			return getRgpDescriptor((RadialGradientPaint) paint, genCtx);
		
		return super.handlePaint(paint, genCtx);
	}

	private SVGPaintDescriptor getRgpDescriptor(RadialGradientPaint gradient, SVGGeneratorContext genCtx)
	{
		Element gradElem = genCtx.getDOMFactory().createElementNS(SVG_NAMESPACE_URI, SVG_RADIAL_GRADIENT_TAG);

		// Create and set unique XML id
		String id = genCtx.getIDGenerator().generateID("gradient");
		gradElem.setAttribute(SVG_ID_ATTRIBUTE, id);

		// Set x,y pairs
		Point2D centerPt = gradient.getCenterPoint();
		gradElem.setAttribute("cx", String.valueOf(centerPt.getX()));
		gradElem.setAttribute("cy", String.valueOf(centerPt.getY()));

		Point2D focusPt = gradient.getFocusPoint();
		gradElem.setAttribute("fx", String.valueOf(focusPt.getX()));
		gradElem.setAttribute("fy", String.valueOf(focusPt.getY()));

		gradElem.setAttribute("r", String.valueOf(gradient.getRadius()));

		addMgpAttributes(gradElem, genCtx, gradient);

		return new SVGPaintDescriptor("url(#" + id + ")", SVG_OPAQUE_VALUE, gradElem);
	}
	
	private SVGPaintDescriptor getLgpDescriptor(LinearGradientPaint gradient, SVGGeneratorContext genCtx)
	{
		Element gradElem = genCtx.getDOMFactory().createElementNS(SVG_NAMESPACE_URI, SVG_LINEAR_GRADIENT_TAG);

		// Create and set unique XML id
		String id = genCtx.getIDGenerator().generateID("gradient");
		gradElem.setAttribute(SVG_ID_ATTRIBUTE, id);

		// Set x,y pairs
		Point2D startPt = gradient.getStartPoint();
		gradElem.setAttribute("x1", String.valueOf(startPt.getX()));
		gradElem.setAttribute("y1", String.valueOf(startPt.getY()));

		Point2D endPt = gradient.getEndPoint();
		gradElem.setAttribute("x2", String.valueOf(endPt.getX()));
		gradElem.setAttribute("y2", String.valueOf(endPt.getY()));

		addMgpAttributes(gradElem, genCtx, gradient);

		return new SVGPaintDescriptor("url(#" + id + ")", SVG_OPAQUE_VALUE, gradElem);
	}
	
	private void addMgpAttributes(Element gradElem, SVGGeneratorContext genCtx, MultipleGradientPaint gradient)
	{
		gradElem.setAttribute(SVG_GRADIENT_UNITS_ATTRIBUTE, SVG_USER_SPACE_ON_USE_VALUE);

		// Set cycle method
		switch (gradient.getCycleMethod())
		{
		case REFLECT:
			gradElem.setAttribute(SVG_SPREAD_METHOD_ATTRIBUTE, SVG_REFLECT_VALUE);
			break;
		case REPEAT:
			gradElem.setAttribute(SVG_SPREAD_METHOD_ATTRIBUTE, SVG_REPEAT_VALUE);
			break;
		case NO_CYCLE:
			gradElem.setAttribute(SVG_SPREAD_METHOD_ATTRIBUTE, SVG_PAD_VALUE);	// this is the default
			break;
		}

		// Set color space
		switch (gradient.getColorSpace())
		{
		case LINEAR_RGB:
			gradElem.setAttribute(SVG_COLOR_INTERPOLATION_ATTRIBUTE, SVG_LINEAR_RGB_VALUE);
			break;
			
		case SRGB:
			gradElem.setAttribute(SVG_COLOR_INTERPOLATION_ATTRIBUTE, SVG_SRGB_VALUE);
			break;
		}

		// Set transform matrix if not identity
		AffineTransform tf = gradient.getTransform();
		if (!tf.isIdentity())
		{
			String matrix = "matrix(" + 
					tf.getScaleX() + " " + tf.getShearX() + " " + tf.getTranslateX() + " " + 
					tf.getScaleY() + " " + tf.getShearY() + " " + tf.getTranslateY() + ")";
			gradElem.setAttribute(SVG_TRANSFORM_ATTRIBUTE, matrix);
		}

		// Convert gradient stops
		Color[] colors = gradient.getColors();
		float[] fracs = gradient.getFractions();
		
		for (int i = 0; i < colors.length; i++)
		{
			Element stop = genCtx.getDOMFactory().createElementNS(SVG_NAMESPACE_URI, SVG_STOP_TAG);
			SVGPaintDescriptor pd = SVGColor.toSVG(colors[i], genCtx);

			stop.setAttribute(SVG_OFFSET_ATTRIBUTE, (int) (fracs[i] * 100.0f) + "%");
			stop.setAttribute(SVG_STOP_COLOR_ATTRIBUTE, pd.getPaintValue());

			if (colors[i].getAlpha() != 255)
			{
				stop.setAttribute(SVG_STOP_OPACITY_ATTRIBUTE, pd.getOpacityValue());
			}
			
			gradElem.appendChild(stop);
		}		
	}	
}

Hello, sorry, I didn't see your comments until now. You can use the code under the MIT license if it is still relevant for you.