Check your php plugins directory.
/usr/lib/php4/20010901/
Or something like that. I dont have gd installed on my machine but I know that if you are getting an error like the one you are getting PHP flat out doesnt know what who or where GD is.
(The plugin should be like gd.so or something)
If you dont have it try downloading it. From rpmfind.net
http://rpmfind.net/linux/rpm2html/search.php?query=php-gd&submit=Search+...
Although I have to put in a shameless plug that had you had Debian installed it it would have been a nice
apt-get install php4-gd
End of story.
Hope this Helps!