You have to change your LPI.
- custom driver - may have an lpi option for this Atleast linotronic used to many years ago)
- software RIP - change LPI at your RIP station.
- color laser printer - you would have to print seperations.
Or to msilaulte this edit you gradient in illustrator: effect >> pixelate >> color halftone, enter a small amount like 12 for max radius.
If your printer is a PostScript printer capable of printing color separations, it may have a so-called "resolution enhancement" feature that is turned on by default. If so, you probably need to turn that off to get it to obey custom halftone settings. I know, for example, many HP laser printers are like that.