Thanks for the feedback.
Gradient Color is not supported for line/arrow.
So we've just build a beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
that ignores the gradient setting in that case.
Hoping that this is enough for you.
BTW, we have implemented a simplified skinparam arrowColor which you should use now (see http://doodle.com/poll/nt23tbuvfp2rmr92