The antonyms for the word "more gibbous" could be "less round", "less swollen", "less bulging", or "less protruding". These words suggest a shape that is more flat, concave, or thinner. They are often used to describe objects that have a smaller and more defined shape, such as crescent moons, thinner bellies, or narrower curves. In contrast, "more gibbous" indicates a shape that is more pronounced or exaggerated in its fullness or roundness. By using antonyms like "less round" or "less swollen", we can create a more precise description of shape and help readers visualize objects more accurately.