As for the grid, maybe you want long_ticks? (or x_long_ticks and y_long_ticks to set them separately)

I'm not sure if you can print the labels slanted (actually, I'm afraid you can't without hacking the source¹), but for long labels there's x_labels_vertical.

___

¹ the idea would be to change $angle in the following snippet:

        if (defined $text)
        {
            $self->{gdta_x_axis}->set_text($text);

            my $angle = 0;
            if ($self->{x_labels_vertical})
            {
                $self->{gdta_x_axis}->set_align('bottom', 'center');
                $angle = PI/2;
            }
            else
            {
                $self->{gdta_x_axis}->set_align('center', 'right');
            }
            $self->{gdta_x_axis}->draw($x - $self->{axis_space}, $y, $
+angle);
        }
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(untested)

Great! Both of these were exactly what I was looking for. Do you know how I can change the y_long_ticks colour or make the line dotted? Thank you, Kevin

I don't think this is configurable, but - as usual - you could modify the sources.  In this case, it looks like changing the following snippet (search for "# Ticks and values for y axes" in axestype.pm)

            if ($self->{y_long_ticks})
            {
                $self->{graph}->line(
                    $x, $self->{bottom},
                    $x, $self->{top},
                    $self->{fgci}
                ) unless ($axis-1);
            }
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might have the desired effect...

The ->line method is from the GD module, so check its docs for what parameters are expected. The $self->{fgci} argument (foreground color) is presumably what you'd need to manipulate. See also ->setStyle for dotted/dashed lines.

The x_labels_vertical => 1 option lets you turn the labels 90°, I don't think other angles are supported.

Yes, both rotating the values and adding a grid is possible with just a few set commands. The following code, modified from Plot a spiral with gnuplot, does both.

use strict; use warnings; use 5.010;
use IO::Handle;
use File::Temp "tempfile";
my($T,$N) = tempfile("plot-XXXXXXXX", "UNLINK", 1);
for my $t (100..500)
        { say $T $t*sin($t*0.1), " ", $t*cos($t*0.1); }
open my $P, "|-", "gnuplot" or die;
printflush $P qq[
        unset key
        set xtics rotate by -45
        #set grid # vertical and horizontal grid lines
        set grid xtics # vertical grid lines only
        plot "$N" with lines lw 3
];
<STDIN>;
close $P;
__END__
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Update: rotating the text is tricky in that some of the terminals (output drivers of gnuplot) does not support it, or only supports rotating by multiples of right angles. This may also depend on the version of gnuplot you have. Rotating the text by any angle does work in at least the postscript driver though.

You're using gnuplot. The question was about GD::Graph. This is a pretty significant difference. (That said, gnuplot is a good tool to use.)