bell gable vs gable wall

bell gable and gable wall both are nouns.

Word	Noun	Adjective	Verb	Adverb
bell gable	Yes	No	No	No
gable wall	Yes	No	No	No

As nouns, gable wall is a hypernym of bell gable; that is, gable wall is a word with a broader meaning than bell gable:

bell gable: an extension of a gable that serves as a bell cote
gable wall: the vertical triangular wall between the sloping ends of gable roof

Other hypernyms of bell gable include gable, gable end.

bell gable (noun)	gable wall (noun)
an extension of a gable that serves as a bell cote	the vertical triangular wall between the sloping ends of gable roof