You probably just need to change the LPI of the dot pattern. If you are printing from a vector, use Classic Dot and set the LPI for 70.7. If you are using an image, use Gerber Tone STC Photo and set the LPI for 70.7.
If that doesn't work, try changing your output DPI to 300 x 300 (if you're printing at 600 or vice versa if you're printing at 300.
If all else fails, in GSPPlot, turn off your automatic color management and manually select the Gerber Edge 2 CMYK 300 DPI color profile from the drop down menu and which is a great all around profile.