Another gotcha with GD fonts, is that it is easier to just copy the desired font to your working directory, to get the path right. Here is a simple example. Your GD must be compiled with FreeType IIRC, to enable ttf font support. But then, you can import any ttf font you like.
in reply to Converting X11 fonts for GD
my ($w, $h) = (200, 200);
my $img = GD::Image->new($w, $h);
my $white = $img->colorAllocate(255,255,255);
my $black = $img->colorAllocate(0,0,0);
# this line causes a premature exit with "success".
#my $monaco = GD::Font->load('Generic.ttf');# or die "Can't load monac
# Generic.ttf is in script's working directory
open (IMG, ">$0.png");
print IMG $img->png;