Noelle, You can tell if it is loaded from the Moodle admin page. There is a button at the bottum of the page labelled php info. Click on that. If you have GD loaded there will be a section (table) with GD info. I usually just do a find and type in GD to see if it is there.
If no tables in the PHP info shows GD support, then you may need a new PHP installation. Usually you cannot add GD support, but have to start from the beginning and install a new PHP build with GD built in.