Since it took me quite some time to resolve this problem, I hope this post will be useful for someone else.
I am using the CAPTCHA gem in my application and always got the following error:
undefined method `stringFT' for GD::Image:Class
It turns out that I was missing the GD libs for ruby. I'm on ubuntu, so a quick sudo install libgd-ruby solved the problem for me.
I also found resources for OS X and Red Hat Enterprise Linux 4:
OS X: http://lukewarmtapioca.com/2007/02/06/gd-on-osx-and-ruby-stringft/ Red Hat: http://involution.com/2007/06/11/problems-with-rubys-captcha-gem/