open sight vs gunsight

open sight and gunsight both are nouns.

Word	Noun	Adjective	Verb	Adverb
open sight	Yes	No	No	No
gunsight	Yes	No	No	No

As nouns, gunsight is a hypernym of open sight; that is, gunsight is a word with a broader meaning than open sight:

open sight: rear gunsight having an open notch instead of a peephole or telescope
gunsight: a sight used for aiming a gun

Other hypernyms of open sight include gun-sight.

open sight (noun)	gunsight (noun)
rear gunsight having an open notch instead of a peephole or telescope	a sight used for aiming a gun