Apparently, there's a difference between thingamajig, widget, gadget and gizmo!
Thingamajigs are typically specialized devices which have a limited number of uses or a single specific use. The term is typically employed by one whose experience with the use of the object is nonexistent or very limited. Regular users of such devices would never refer to them as thingamajigs or any of the related terms listed below.
A thingamajig is different from a widget, in that a widget is an actual, but not yet named or constructed, mechanical component. It is also different from a gadget, in that “gadget” is the generic term for a superfluously useful device, such as a remote garage door opener, whose name is easily remembered.
Even among the world of otherwise nameless things referred to by placeholder names, there is a hierarchy of specificity. "Thing", as its name implies, is universally applicable. It is likely, however, that a "gizmo" involves some minor degree of technological sophistication, connoting as it does some mechanical or electronic aspect.