The slaty gnateater is long, and 57 specimens had a mean weight of . The plumage is variable, but the typical male's head is mostly dark gray that continues through most of the underparts. The crown and upper parts are brown and the flanks and lower belly are brownish gray. It has a white tuft behind the eye. Birds in the Cochabamba Department of Bolivia are much darker. The female has more brown on the face, the white tuft is much smaller, and there is a white patch on the belly. The upper parts can be dark brown, olive brown, or rufous brown.
