gambo

Esperanto has two words for a leg, you have already learned the other one, and the honest answer is that they overlap.

Here is the dictionary's division. Gambo is the entire lower limb of a person, and the definition spells out the parts it contains: the thigh (femuro), the knee (genuo), the shank (kruro), the foot (piedo). Kruro has two entries: in ordinary use it is the whole limb as well, and in anatomical use it is narrowed to the part from the knee down. So kruro is the loose everyday word, gambo is the precise word for the whole limb, and the technical kruro is the shank inside it.

What that means when you write. Kruro sits at rank 837 and gambo at 2079, and that gap tells you which one Esperanto speakers reach for: kruro is the default, and nobody will misunderstand mi rompis la kruron. Use gambo where precision earns its keep - anatomy, medicine, describing an injury exactly - and where the whole limb from hip to floor is the thing you mean. A table has kruroj, not gamboj; the dictionary gives the furniture sense to kruro.

This is not a flaw. Every language ends up with a common word and a careful word for the same body part, and English does the same thing with leg against limb. Esperanto just has the courtesy to define which is which.