First you prepare your plot as you want it in gnuplot. 
Then:

gnuplot> set term postscript     (will produce postscript output)

gnuplot> set output "printme.ps" (output will be written to the file 
		  		  printme.ps, or any name you use)

gnuplot> replot                  (recreates the plot but you don't 	
                                  see it, it goes to the file)

gnuplot> set term x11            (resets the normal gnuplot
                                  so you can do more plotting)

Now you have a postscript file named  printme.ps  containing 
your plot.  

You can view it 
    with ghostscript:      or     with ghostview:
    UNIX> gs printme.ps           UNIX> ghostview printme.ps

    or print it:
    UNIX> lpr printme.ps