This attribute is used to insert a new gadget into a list of gadgets
linked by their Gadget.NextGadget field.  When the OM_NEW method creates
the new gadget, it inserts the new gadget into the list following the
GA_Previous gadget.  This attribute is a pointer to the gadget  (struct
Gadget *) that the new gadget will follow.  This attribute cannot be used
to link new gadgets into the gadget list of an open window or requester,
use AddGList() instead.