[2] The body of Milax gagates is an even dark grey to black, although it has somewhat lighter sides.

[3] The weight of adult slugs ranges from 991.2 mg to 3308.0 mg.[4] Reproductive system: The penis is rounded, and half as long as the epiphallus.

[3] The stimulator is narrow, conical, slightly flattened at its end with some papillae.

[3] The stimulator of the similar species Milax nigricans has more papillae situated at its base and not near its free end.

[3] Milax gagates has been introduced in the area between Portugal and Galicia, the Atlantic coastlands of France to Belgium, the British Isles, and in many other areas almost worldwide (Australia, Tasmania, New Zealand, Japan, Pacific islands, South Africa, Atlantic islands).

[3] As an introduced species it now occurs in a number of countries and islands including: Milax gagates lives mostly in cultivated areas, often the coast, and also in forests, shrublands, and natural meadows.

[3] It is an agricultural pest on soybean, sunflower, and oilseed rape in Argentina.