"Gizmo" is a brand of polyethelene bottle that screws into the bottom of the skimmer for winterizing. If ice forms in your skimmer the "Gizmo" is supposed to crush to prevent the ice's expansion from bursting the skimmer.

Threads are normal at the bottom of a skimmer. You can put a threaded return plug in it to seal the skimmer off (say, when you are emptying the basket directly on the pump.) You leave it open during normal running.