The GD README states that you need to install the FreeType font rendering library as well as the other module prerequisites prior to installation to enable TrueType font support. The documentation also provides an example for calling stringFT():

 $gd->stringFT($black,'/dosc/windows/Fonts/pala.ttf',40,0,20,90,
              "hi there\r\nbye now",
              {linespacing=>0.6,
               charmap  => 'Unicode',
              });
[download]