php-gd is the GD Library for PHP: https://bitbucket.org/pierrejoye/gd-libgd/src
Downloading and building php-gd
First, update the latest MineOS scripts from the web-ui. Then, run the addon script:
cd /usr/games/minecraft/addons ./php-gd.sh
The GDlibrary is not explicitly called, but simply depended on by other PHP frameworks. The addon script automatically integrates with the PHP configuration and will work when called upon automatically. You will typically only need to download this addon a PHP script says you need it.