Sorry I should have explained. First make the shape you want to use for the gradient, I made a smaller square inside a larger square and then used PATH>>DIFFERENCE to leave a cut out square. Then blur the shape and use another square to tidy up the edges by clipping them together.

I use a cutout square and add blur to produce a pseudo-square-gradient effect. I'll attach an example and .png (for some reason it doesn't show up properly on the svg when viewed in an image viewer.) http://www.inkscapeforum.com/download/file.php