Like other Grallaria, Giant Antpitta has a plump body, short tail, and long legs. Compared to other antpittas, the bill of Giant Antpitta is especially thick and heavy. The forecrown and lores are deep tawny buff or chestnut, and the crown olivaceous brown, grading into a gray hindcrown and nape. The back is olivaceous brown, with the wings and tail browner than the back. The sides of the head and entire underparts are deep rusty buff, barred with broken black bars; the central belly and undertail coverts are deeper colored and unbarred. Juvenile not well described, but Wetmore (1945) noted that the "plumage of the first year" of the nominate subspecies bears rufescent and black feather-tips and bars on the wing coverts, back, and rump. Hilty and Brown (1986) also note that immatures "show a few tawny rufous and black bars on mantle".
Giant Antpitta is among the largest in the genus, and is most similar to Undulated Antpitta (Grallaria squamigera) and Great Antpitta (Grallaria excelsa). With the latter it has no geographic overlap. Above, Giant Antpitta is similar to the slightly smaller Undulated, but is darker and browner with a more rufous forecrown and a slaty crown and nape that are distinctly different than the olive brownish back (compare with the nearly uniform slaty upper parts of Undulated Antpitta). The underparts of Giant Antpitta are much more deeply colored overall: orange rufous, barred with black. Undulated Antpitta has more washed out coloration below, appearing pale buffy, and the black barring is thicker and somewhat more "scaled" looking. Overall, the Giant Antpitta is stouter and has a heavier bill.
Sexes similar. The following description is of hylodroma, based on Fjeldså and Krabbe (1990). See also Geographic Variation.
Adult: The forecrown is rufous chestnut, fading to slate gray on the crown and nape. The remaining upperparts are olivaceous brown. Face, auriculars, and underparts deep rufous chestnut, narrowly barred with black. Barring is heaviest on the chest, becoming finer lower and on the flanks, with the central belly and vent unbarred and slightly paler.
Iris: dark brown
Bill: maxilla blackish; mandible dark gray brown to blackish or somewhat grayish horn
Tarsi and toes: gray to bluish gray
Total length: 24 cm (Ridgely and Tudor 1994, Ridgely and Greenfield 2001b, Krabbe and Schulenberg 2003), 26.7 cm (Hilty and Brown 1986)
wing length: range 144.2-152.6 mm, mean 148.3 mm; tail length range 54.5-61.8 mm, mean 57.0 mm; bill length (culmen from base): range 38.7-40.0 mm, mean 39.4 mm (hylodroma, n = 4, type series; Wetmore 1945); tarsus length: range 66.3-70.6 mm, mean 69.7 mm (hylodroma, n = 3, type series; Wetmore 1945) – the type of G. g. hylodroma (male) designated by Wetmore (1945) measured: wing 152.6 mm, tail 61.8 mm, culmen from base 38.7 mm, tarsus 70.6 mm.
wing length 153 mm; tail length 67 mm; bill length 39 mm, tarsus length 58.5 mm (hylodroma, n = 1; Ménégaux and Hellmayr 1906).
wing length 147.3 mm; tail length 61.3 mm; bill length (culmen from base) 39 mm; (hylodroma, n = 1; Wetmore 1945)
sex unknown –
wing length 142.2 mm, tail length 54.3 mm, bill length (culmen from base) 39.4 mm, tarsus length 69.8 mm (gigantea, holotype; Wetmore 1945)
wing length 154.1 mm, tail length 60.7 mm, bill length (culmen from base) 38 mm, tarsus length 73 mm (lehmanni, holotype; Wetmore 1945)
wing length 165 mm, tail length 80 mm, bill length 32.7 mm, bill width at base 14.8 mm, bill at nares 25.5 mm, bill width at nares 11.6 mm (hylodroma, n = 1; de Soye et al. 1997)
male: 204–266 g (gigantea, n = 3; Krabbe and Schulenberg 2003); 266 g (n = 1, gigantea, Krabbe et al. 1994); 218 g (n = 1, hylodroma, Krabbe et al. 1994)
sex unspecified: 254 g (hylodroma, n = 1, de Soye et al. 1997)