Add Blocks to the Creative Gui

import java.util.List;

ModLoader.SetInGUIHook(this, true, false);

boolean addedToGui;

public boolean OnTickInGUI(float f, Minecraft minecraft, GuiScreen guiscreen)
    {
        if ((guiscreen instanceof GuiContainerCreative))
        {
            if (!addedToGui)
            {
                Container container = ((GuiContainer)guiscreen).inventorySlots;
                List list = ((ContainerCreative)container).itemList;
                list.add(new ItemStack(guiBlock, 1));
                addedToGui = true;
            }
        }
        else
        {
            addedToGui = false;
        }
        return true;
    }