Watch. Move the mouse around. Enjoy. Remix.
Featured! :D I added automatic resolution adjustment. (inspired by zorket's remix) The gradient itself is generated by passing a number to <set pen color to ()>, which interprets it as a color of the form 0xRRGGBB. This is done inside of a loop which divides the stage into n×n "pixels" and draws a dot at each one. It uses the <direction> block and some math to do the animation.