Mammoth Uncensored Word List:
25 - 13 letter words starting with go

goatherdesses goatishnesses gobbledegooks gobbledygooks godlessnesses godlikenesses goldenberries goldsmithries gollywhompers gonadectomies gonadotrophic gonadotrophin gonadotropins goniometrical goodheartedly goodnaturedly goosestepping gormandisings gormandizings gossipmongers gourmandising gourmandizing governability governmentese governorships