straight face vs facial gesture

straight face and facial gesture both are nouns.

Word	Noun	Adjective	Verb	Adverb
straight face	Yes	No	No	No
facial gesture	Yes	No	No	No

As nouns, facial gesture is a hypernym of straight face; that is, facial gesture is a word with a broader meaning than straight face:

straight face: a serious facial expression giving no evidence of interest or amusement
facial gesture: a gesture executed with the facial muscles

Other hypernyms of straight face include facial expression.

straight face (noun)	facial gesture (noun)
a serious facial expression giving no evidence of interest or amusement	a gesture executed with the facial muscles