I am not affiliated with either of these apps, but if you are going to buy this one trick pony, then I suggest paying just a little more and getting an app that can do more than just gradients:
http://itunes.apple.com/us/app/css3-toolkit/id479856901?mt=12
Gradient is nice looking, but only does one thing. That one thing you can do for free (and actually better because Gradient does not support stops right now) by going to this site: http://www.colorzilla.com/gradient-editor/