Here's how I resolved the "Fatal error: Call to undefined function: imagecreate()" error using Gentoo:

1) add a USE flag for gdb in /etc/make.conf
  
   USE="3dnow avi [whatever else you have] gdb"

   Note: here's a list of all of the USE flags:
   http://www.gentoo.org/dyn/use-index.xml

2) unmerged mod_php 

*Note* It could take awhile to "remerge" as it may need to compile several dependancies...do this during not production hours and have a backup if you absolutely cannot have downtime

   emerge -C mod_php

3) emerged mod_php

   emerge -p mod_php 
    # find out if it's going to take awhile

4) edit /etc/php/apach2-php4/php.ini
     uncomment the "extension=php_gd2.dll" line

5) Restart apache2

    /etc/init.d/apache2 restart

Hope this helps!

--
Cooper Stevenson
GenCom
http://www.gencom.us