Mammoth Uncensored Word List:
21 - 17 letter words starting with g

galactopyranoside galvanostalametry galvanosurgically gastroenteritides gastroenteritises gastroenterostomy gastrojejunostomy gentleheartedness gentlemanlikeness gentlemanlinesses glossopharyngeals glycophospholipid glycosaminoglycan glycosphingolipid glyphographically glyptographically goodheartednesses governmentalizing grammaticalnesses greatgrandmothers gynandromorphisms