Problem/Bug How to put items in a custom GUI?

Hello users of the Bukkit forum.
I am working on my plugin, all the things going smoothly for my first plugin, until I created my custom GUI and an item for it. I don't know how to add the item to the GUI

Here is the entire structure I explained above:
http://pastebin.com/ybNZZ5Da

On the line 8 of the code, is this method "p.getInventory().addItem(head);". If you don't understand, the code will get the player's inventory and add the ItemStack "head", but I want to get the custom GUI instead of the player's inventory.
Can someone tell me what to modify?

Nevermind, I resolved it

EDIT by Moderator: merged posts, please use the edit button instead of double posting.