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The longest ones I could find in Webster's Second International dictionary are 17 letters long: antiagglutinating autoagglutinating
Another word starting with g is gloominess
The first adjective in the dictionary starting with the letter "g" is "gabbroic." It refers to something that is related to or resembling gabbro, a type of coarse-grained igneous rock. This adjective is often used in geological contexts.
gabbroic
gynotikolobomassophile
There are two at 21 letters each, but looks like gastroenteroanastomosis wins at 23 letters: 23 gastroenteroanastomosis 21 gastroenterocolostomy 21 glossolabiopharyngeal
The longest word in the English language beginning with the letter G is "gastroenterocolitises," which is a plural form of "gastroenterocolitis." Gastroenterocolitis is the inflammation of the stomach, small intestine, and colon. The plural form refers to multiple instances of this condition.
killing
good
Geometry.
gloomy
global