Gymnocalycium mostii is a solitary cactus with dark green, flattened spherical stems, growing 6–7 cm tall and up to 13 cm in diameter. It has 11–14 deeply notched ribs divided into humps. The strong, curved spines are yellowish-brown with darker tips, aging to gray. There are 1–2 central spines up to 2 cm long and 7–11 radial spines ranging from 0.6 to 2.2 cm. The flowers are pink to white, sometimes with a darker throat, and measure up to 8 cm in length and diameter. The egg-shaped fruits are slate to blue-green, up to 1.5 cm in diameter and 2 cm long.