Ok, in X-Cart, you never really need to touch the php. All of the files you need are in the templates. For the gift registry, all of the files are in the skin1/modules/Gift_Registry folder. Simply use Webmater Mode to find out which template you are looking for, and then open that template to see the form code for the part you are looking for. May want to read this thread to give you some guidance on the template system in X-Cart:
http://forum.x-cart.com/showthread.php?t=21529