To make multi-stop gradient, simply make several linked offsets of different colours and sizes (3). Because the inner path is linked to the outer one, any changes to the outer path will be updated in the shape of the inner one (though you may need to readjust the blur radius if you change the shape radically). Use the node handle to make it smaller than the original path (1) and blur it to create the gradient (2). Then make 'Path -> Linked Offset', raise it to the top and set it to the inner colour (or usually somewhat more saturated one, to compensate the mixing when blurred). To make a 'radial' gradient of any shape, that is, a gradient that changes from the edge of the path toward the centre, first create the path and set its fill colour to what you want the outer colour to be. When you blur any object different in colour than its background, you are essentially creating a shaped gradient between those colours.

To make gradients of other shapes, the Gaussian blur tool becomes handy.

For example, the radial gradient is limited to circular or oval in shape. The gradient tool in Inkscape is incredibly useful, but it has its limitations as well. Note: This tutorial was written for Inkscape, but should be applicable for other vector drawing programs with similar functionality.

Inkscape tutorial for blur