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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
Oh, dude, the longest word in the English language starting with G is "gastroenteritis," which is like a fancy term for a stomach bug. So next time you're feeling all queasy and stuff, you can impress your friends by dropping that word like it's hot. Just make sure you're not too busy running to the bathroom to show off your vocab skills.
killing
good
Geometry.
gloomy
Ghoul.