Mammoth Uncensored Word List:
20 - 18 letter words starting with g

gastroduodenostomy gastroenterologies gastroenterologist gedankenexperiment generalizabilities geochronologically geomorphologically geosynchronousness glomerulonephritis glomerulosclerosis glottochronologies glucocerebrosidase glycosaminoglycans glycosphingolipids grammaticalization granulocytopoieses granulocytopoiesis greatgrandchildren greatgranddaughter greatheartednesses