Did you read the docs, http://search.cpan.org/~mverb/GDGraph-1.43/Graph.pm#USAGE?

No?     Oh, sorry :-(
 

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Methods for charts with axes.

$graph->set_x_label_font(font specification)
$graph->set_y_label_font(font specification)
$graph->set_x_axis_font(font specification)
$graph->set_y_axis_font(font specification)
$graph->set_values_font(font specification)

    Set the font for the x and y axis label, the x and y axis value la
+bels, and for the values printed above the data points. See "FONTS".
[download]

Yeah, may or may not do precisely what you want, but finding that took only a few seconds and there's a rather large part of the doc I haven't read.

But my cursory effort also uncovered this which may even offer some code to cargo-cult.