In attachment management, gd or imagemagick are only used for
generating the thumbnail of attached images. There is no "image manipulation" ...
Why do you refuse imagemagick? It has much better algorithms for resampling images than gd, it keeps meta information(for example exif data) for the thumbnail, keeps animation in gifs etc.
If you want to build a extension, you can decide if you want to use gd-function (those are php-commands like "image..." ->
http://php.net/manual/en/book.image.php ) or imagemagick, which is a separate set of binaries, which are executed via shell-command.