Gassendi est une grande plaine murée à fond fracturé de 110 Km et profond de 1860 m.  Situé sur le bord nordique de la mer des Humeurs (MareHumorum). La formation a été inondée par la lave pendant la formation de la mer, ainsi seul la muraille et les crêtes centrales multiples (1200 m) demeurent au-dessus de la surface. Les murs externes sont érodés. Un cratère Gassendi A (33 Km / 3600 m) a impacté le mur nordique. On note aussi un soulèvement relatif de la partie nord-ouest du plancher. La partie méridionale  plonge vers le bas et le mur ne mesure  que  200  mètres  contre  2.5 kilomètres pour le mur le plus haut. Le  fond est tourmenté avec de nombreux monticules. Il y a également un système de rainures entrecroisées, appelé Rimae Gassendi.

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Gassendi is a large lunar crater feature located at the northern edge of Mare Humorum. The formation has been inundated by lava during the formation of the mare, so only the rim and the multiple central peaks remain above the surface. The outer rim is worn and eroded, although it retains a generally circular form. A smaller crater 'Gassendi A' is intruding into the northern rim, and joins a rough uplift at the northwest part of the floor. The crater pair bears a curious resemblance to a diamond ring.

In the southern part of the crater floor is a semi-circular ridge-like formation that is concentric with the outer rim. It is in the southern part where the rim dips down to its lowest portion, and a gap appears at the most southern point. The rim varies in height from as little as 200 meters to as high as 2.5 kilometers above the surface. The floor has numerous hummocks and rough spots. There is also a system of rilles that criss-cross the floor, named the Rimae Gassendi.

On some older maps the 'Gassendi A' crater was called "Clarkson", but this name is not officially recognized by the IAU.