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        on unix
  or:
gnuplot> set term win        on win  
		(resets the normal terminal 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:           or     produce .pdf:
    UNIX> lpr printme.ps	  UNIX> ps2pdf printme.ps printme.pdf 

gnuplot> set term png             (will produce .png output)
gnuplot> set output "printme.png" (output will be written to the file 
		  		  printme.png, or any name you use)
gnuplot> replot
gnuplot> set term x11

You can view it with:  xv printme.png   or  on a browser  or print it...
or import it in PowerPoint  or  whatever you want to do with it.