The Merriam-Webster dictionary reads that a gadget is “an often small mechanical or electronic device with a practical use but often thought of as a novelty”.
The term “gadget” has no sure etymology although it is believed that it comes from a French company by the name of Gaget. That company was making small replicas of the Eiffel tower in order to promote and finance the project of Mr. Bartholdi (i.e. the Eiffel tower, of course).
A gadget is really often thought of as an item that is ingenious, original, maybe fun, with – perhaps – an interesting design but that is not very useful. In a word, people are dubious about the effectiveness and usefulness of a gadget.