GUI that does not clear it's items on close/open, any external non GuiItem added to it will stay. (it does not persist on server restart)
To create a persistent GUI all you need to do is:
StorageGui gui = Gui.storage()
Differently to the normal GUI, the
addItemmethod takes an
GuiItemadded will have actions applied to it, the persistent items are simple ItemStacks that have nothing associated to it.