gradient <- linearGradient(c("black", "white", "black"), 
                           x0=0, y0=.5, x1=1, y1=.5)


grid.rect(name="r")


gradient <- linearGradient(c("black", "white", "black"), 
                           x0=0, y0=.5, x1=1, y1=.5)


grid.gradientFill("r", gradient)


grid.export()