currently this is not possible. It might be added, but then probably only for HTML5, trying to simulate the HTML5 image gradients also in Flash might be laborious and slow in rendering...
A kind of gradient shading could be already achieved (in HTML5) by using the bgshadow setting with the 'inset' style and larger blur and spread values: https://krpano.com/docu/xml/#layer.bgshadow