From what I can remember GD is installed with PHP 4.3.2. As for functionality GD is much simpler than ImageMagick. ImageMagick supports loads of image formats and can do lots of filters and effects on the images. If you want to the same with GD you would have to program it in PHP.
All image conversion are configured by the settings/image.ini file.
You should take a look at the Rule group and change any occurence of 'convert' to 'gd'. You might also want to turn on HasGD2 in GDSettings if you have the newest GD version (which I believe is shipped with PHP 4.3.x).