Shell large and solid with markedly stepped profile and a row of tubercles on the step below each suture; last whorl keeled; umbilicus obvious; marked with irregular red streaks and spots. There are 7-8 whorls, the last one rather sharply keeled, and the sutures are usually deep. In addition to the tubercles, of which there are about 20 on the last whorl and 14-18 on the two previous, the shell bears spiral ridges, one of which is enlarged to form the keel. There are commonly 17-18 ridges on the adapical half of the last whorl, often alternately large and small, and 22-27, more equal in size, on its base. The penult whorl has 12-14. the others 6-8 each. The number is variable and ridges on the oldest whorls and on the shell base are often eroded; the ridges may be spiky in shells from deep water. The umbilicus is large, round and deep, approached by a comma-shaped groove. There is a distinct bulge on the columella. Up to 30 mm high, 35 mm broad; last whorl occupies 70% of shell height, aperture 40%-.
The snout is densely papillated distally and has a ventrolateral fringe on each side. The cephalic lappets are large, each with a lobed margin, and connect over the tentacle base to the eye stalk; the right eye stalk is joined to a neck lobe with a smooth edge; the left neck lobe is not so joined and has a scalloped edge. Each neck lobe is linked to an epipodial ridge which runs along the foot; ventral to this on each side arise three epipodial tentacles, their bases in sheaths with papillated edges and with a spiky flap-like sense organ. The foot is blunt anteriorly, pointed posteriorly where its dorsal surface carries many transverse grooves. The flesh is yellow-pink or orange with black-purple blotches, similar lines over the snout and front of the foot, a central line with branches on each tentacle; epipodial tentacles are sulphur yellow, their sheaths and sense organs white; neck lobes and cephalic lappets yellow.

G. magus extends north from the Mediterranean and the Azores to the British Isles, where it has been found in the Channel as far east as Swanage. the Cotentin peninsula and the Channel Islands. It occurs all round Ireland and north to Shetland on the west coast of Britain; not in the North Sea except perhaps very occasionally. The animals like the same kind of shore as other top shells but usually live below tidemarks. though rare specimens may be found at L.W.S.T. Their mode of life is like that of other Gibbula spp. Eggs are laid singly in spring and early summer, each in a jelly coat. They hatch to give trochophore larvae which settle after a brief free life.