I started getting slightly obsessed/hypnotized with gradient animations and wanted a tool to see what I could do, so I built one.  Basically, I just bound a bunch of sliders to the various transforms on the brush transform.  I reused the infrastructure from an earlier post. 

As you watch the animation and see something you like, you can pause it and note the values for your own purpose.  I didn't actually create an exporter for the xaml, but that wouldn't be hard.  Download the source or run the XBAP.