Description (holotype): Shell elongate-biconic, strong, medium-sized, slightly glossy, covered by thin light yellowish smooth periostracum. Protoconch eroded, brown, multispiral, off. 2.6 whorls, diameter 670 µm. Transition from protoconch to teleoconch clearly marked by change in colour. Teleoconch of 9.75 low whorls separated by shallow-channelled suture. Whorls bear well-developed subsutural fold and peripheral keel. Subsutural fold appears on first teleoconch whorls and is narrower than keel, although its relative width progressively enlarges with shell growth. Subsutural fold covered by rounded blunt gemmules, which occupy whole fold on early whorls, but on last and most of penultimate whorl are confined to narrowing cord in middle of fold which becomes more flat on last whorl; there are 39 gemmules on last whorl and 28 on penultimate whorl. On seventh teleoconch whorl the cord at upper edge of subsutural fold appears and over the extent of one whorl this cord is split in two, which become more convex and well developed on last whorl. Peripheral keel bears longitudinally elongate gemmules that are arcuate on last and penultimate whorls; there are 36 gemmules on last whorl and 28 on penultimate whorl. Interspace between subsutural fold and peripheral keel is very narrow and smooth on upper 5 teleoconch whorls; later there appears an initially narrow cord, becoming progressively broader and more pronounced. On last whorl the interspace between fold and keel is broad with three narrow but distinct spiral cords. Body whorl occupies 0.61 of shell length. Periphery of whorl below keel, shell base and canal are covered by narrow granulated cords slightly differing in width. There are in total 22 such cords, with interspaces not exceeding cord width. Aperture is narrow and its width slowly decreases to a broad and obliquely truncated canal. Inner lip smooth and convex in its parietal part and nearly straight in columellar part; covered with thin oil-white glossy callus. Weak columellar pleat encircles columellar obliquely. Outer lip projects strongly forward below anal sinus; sinus is deep, U-shaped, slightly adapically directed. Shell length 25.3 mm, body whorl length 15.6 mm, aperture length 8.5 mm, canal length 3.0 mm and shell diameter 8.5 mm.

Although there is clear conchological similarity among all specimens that are attributed to the new species on the basis of molecular data, G. clandestina is rather variable in terms of shell outline, especially in terms of shell slenderness. The type locality is remarkable in this respect, since the variability within the type series equals the maximal variability within the species (shell width/shell length ratio varies from 0.34 to 0.40, average 0.36 ± 0.01, n = 24). The radula was examined in three specimens, including two paratvpes. The radula is formed of about 48 (paratype MNHN 41943) to 60 rows of teeth (MNHN 41934), 12-20 nascent, 2.51-2.87 mm in length (0.32-0.34 of AL). The marginal teeth are duplex, about 105-137 µm long (1.75-1.37% of AL) and the central formation is rather short, with the anterior border indistinct and fused with the membrane. The cusp is strong and curved in profile.
The species is extremely similar conchologically to G. moosai and some specimens cannot be distinguished even by morphometric analysis. For discussion and comparison see Remarks on the previous species. The holotype of the new species is distinguished from the holotype of G. moosai in having a smaller and broader shell. Both species can be readily distinguished by CO I sequences.

Type locality. E of Luzon Island, Philippines, Philippine Sea, 15°58'N, 121°49.1'E, 422-431 m (Expedition Aurora, stn CP2658).
The species has so far been found only in the Philippines, at depths of 342-542 m.