This immediately put me in mind of 'gimcrack' I'd suspect some invention or confusion involving gewgaw and gimcrack in the mind of the author, although we may be looking at an obscure dialect word.
Breaking it down into component parts:
Gim, as in 'gimcrack', we have the OED:
gimcrack gimcrack, n. and a.(ˈdʒɪmkræk)
Forms: α. 4 gybe-, gibecrake, 7 jibcrack. β. 7 gimcracke, gincracke, 8 jem-, jimcrack, 9 dial. gimcrank, 7– gimcrack.
[The 14th c. form gibecrake is perh. connected with OF. giber to shake (see jib v.); the primary sense may have been ‘a slight or flimsy ornament’. (For the change to the nasalized forms, cf. mod.F. regimber = OF. regibber to kick.) The second element may be connected in some way with crack n. or v. Sense 3 is perh. in part due to association of the word with gim a. and crack n. 11 and 14.]
App. applied to some kind of inlaid work in wood. Obs.
1360 Acc. William de Rothewelle in Pipe Roll No. 204 m. 42 d, Et Eustachio de Glastonia..in precio j tabule cum j piler et Gibecrake bordura cum minutis peciis diversi coloris..j ta ula de quercu j piler et Gybecrake de Buxo, xvjs. viijd...
a. A fanciful notion; also, a ‘dodge’, underhand design (obs.). b. A mechanical contrivance; also pl. scientific apparatus. c. Now usually applied to a showy, unsubstantial thing; esp. to a useless ornament, a trumpery article, a knick-knack.
1635 Shirley Coronat. ii. (1640) D ij, Such spectacles Are rare ith' Court, and they were to skirmish naked Before her, then there might be some excuse, There is some gimcrackes in 't, the Queen is wise Above her yeares. 1639 Chapman & Shirley Ball iv. H iij, Luc. There remaines to take away one scruple. Co. Another gimcracke. Luc. I have none, tis your doubt sir. 1676 Walton Angler i. xxi. (ed. 5) 263 Ribbins, and Looking-glasses, and Nut-crackers, and Fiddles, and Hobbyhorses, and many other gim-cracks..and all the other finnimbruns that make a compleat Country Fair. 1709 Steele Tatler No. 34 ⁋5 My Eye was diverted by Ten Thousand Gimcracks round the Room...
So we have a plausible path from 'gim' back to 'jib' or 'gybe' in the sense (according to the OED) of 'swinging around'.
As for 'gew', we have to run the gauntlet of the OED's havering about the origin of the term 'gewgaw'. 'Gaw' is uncontroversially something showy that attracts attention (as in Gawk) but 'gew' is a puzzle unless one speculation (in the OED) that it is simply a doubling of 'gaw' is correct. Thus we would have 'gaw-gaw' (modified to 'gewgaw') just as today we have 'bling-bling'.
Putting it all back together, 'gimgew' in the novel might derive from the components 'gybe' and 'gaw' which suggest a showy ornament that sways or moves. Whether this so-called derivation is real, accidental, or contrived is a question only more research, or a thorough examination of the author's style might answer. One might ask whether the author has a fondness expressed in other examples of their work for word-play, or whether they might be a philologist writing novels in their spare time - as has happened previously of course.