The flower color is listed as pink, but to me its a little more on the apricot side. G. mostii is from the area of Córdoba in Argentina where it grows among grasses in the mossy cracks and cavities in rocky cliffs. The plants in habitat are different from its neighbors. Some have strong spines others weak, curly, straight, long or short ones and in all different combinations. 
They have been described as solitary, but they can also form groups with age. It is a summer grower species that offers no cultivation difficulties. Water regularly in summer and keep this plant almost dry in winter at a minimum temperature of 32 degrees.
From the Greek words 
Gymnos live in various habitats and elevations in Argentina, part of Uruguay, Paraguay, southern Bolivia and part of Brazil which is the South-Eastern part of South America. The largest of this genus may achieve 7 inches in height and 12 inches in diameter, while most of the species stay well under 5 inches in height and diameter. This small size have relatively easy care requirements making them very popular in cultivation. These flowers are naked without any spines, wool, or bristles. In all species, they are smooth and scaled, resembling the tips of young asparagus shoots. Nearly all species have white, cream-colored or pale-pink flowers, while a few will have dark red or yellow. Flower size is moderate and typically flowers are 1 to 1.5 inches wide. While flowers come easy and at an early age on Gymno plants, they usually need high-heat to open fully and thus do much better in a greenhouse.
Gymnocalyciums in habitat are found in South-Eastern South America in various environments and elevations.
The Gymno. achirasense grows in Argentina in the foothills of the city of Cordoba. The variety name "orientale" means that it lies in the most eastern part of the habitat. Achirasense is a (usually) solitary cactus, flattened or globular, dark-green, grey-green to olive green, 2"-3" in diameter up to 2" in height, but in cultivation it can become 4" high (or more) over the years. 10-12 spines which are stout and aggressive, but usually fewer in young plants, bent against the body with a rib count of 12-15. Central spines are usually absent or they have 1 erect. The flowers are some of the largest in this genus, often growing larger than the plant, whitish, pale pink to lavender, with a darker mid-strip.The cultivation of this plant as well as other Gymnocalyciums is simple. They are summer growers, water regularly in summer, keep rather dry in winter. They will flourish in full sun (not direct) or light shade. The Gymnocalyciums are one of the easiest of the cacti to grow, to have flower and can flower at an early age. 
The genus name gymnocalycium comes from the Greek for naked calyx This is a very slow growing species from the grassy hillsides of Serria Grande of Cordoba, Argentina at an altitude of about 2500 feet. Generally solitary, older plant specimens may cluster from the base. The body is flattened with a green-blue or dull grey-green color reaching a width of about 3 1/2" and about 1 1/2" in height. 1, 2, or 3 spines that are stiff and bent to the body. The most characteristic vatteri form is the one with a single spine. Flowers are white with light pink throat and about 1 1/2" long. Named after Ernst Vatter of Buenos Aires.
The plant above in the collection is in a 3" pot.
OLD MAN OF THE MOUNTAIN
