Gimcrack evidently means ‘a showy object of little use or value’ and first came into English usage in 1676 (I love how Merriam-Webster is always so precise about dates, but never offers any evidence). Gew gaw, on the other hand, and according to OED, came into use as early as the thirteenth century, related to French ‘gogue’ – rejoice, play, prank, game – or, perhaps, from ‘jou-jou’ a baby-talk work from French ‘jouer’ (to play) from Latin ‘jocare’. The Italians say ‘giocare’, of course, the root for Leonardo’s smiling Mona Lisa – ‘La Gioconda’ – jocund – ever amused by her foolish audience of voyeurs.
The fact that ‘gimcrack’ arose in the seventeenth century, at the same time as the proliferation of Wunderkammers and cabinets of curiosities, jammed with luxurious, wondrous but often useless gew gaws (unicorn horns that were probably narwhale horns, ossified baby birds, shells footed in gold and silver to serve as awkward vessels for perfumes and oils) is probably significant. By the nineteenth century, English developed the term ‘curio’ to signify gimcracks specifically from the ‘Far East’ – the exotic wonders of Japonisme. The nineteenth century also invented ‘bric-a-brac’ from French ‘a bric et a brac’ meaning random or ‘any old way’ – giving rise to the stuffed study full of gimcracks one associates with literary characters like Sherlock Holmes.
Gimmick might derive from gimcrack (which, by the eighteenth century took on the meaning of a mere ‘trifle’), although it might also be an anagram for ‘magic’, not at all a trifling matter.
evidence). gewgaw (n.) early 13c., giuegaue, contemptuous reduplication, possibly connected with Old French gogue “rejoicing, jubilation; joke, prank, mockery, game;” or jou-jou “toy,” baby-talk word, from jouer “to play,” from Latin jocare (see joke).