Like what iSign said print it as RGB not CMYK. I just notice this myself. I am used to start working on CMYK never on RGB coz most of my work includes adjusting flesh tones and its much easier for me to adjust on CMYK mode. MIne has either red or green. But I dont see them anymore on any grey gradient. Also what also Graphixtreme says sometimes how your rip software handles the color. If your project that you're having problem with isn't not banner or its not a huge project try running it on high quality. Im pretty sure that the gradient will come out smooth.