When -gray is selected, gnuplot honors the following resources for grayscale or color displays (shown here with their default values). Note that the default background is black.
gnuplot*background: black gnuplot*textGray: white gnuplot*borderGray: gray50 gnuplot*axisGray: gray50 gnuplot*line1Gray: gray100 gnuplot*line2Gray: gray60 gnuplot*line3Gray: gray80 gnuplot*line4Gray: gray40 gnuplot*line5Gray: gray90 gnuplot*line6Gray: gray50 gnuplot*line7Gray: gray70 gnuplot*line8Gray: gray30