"end of a ridged roof cut off in a vertical plane, together with the wall from the level of the eaves to the apex," mid-14c., "a gable of a building; a facade," from Old French gable "facade, front, gable," from Old Norse gafl "gable, gable-end" (in north of England, the word probably is directly from Norse), according to Watkins, probably from Proto-Germanic *gablaz "top of a pitched roof" (source also of Middle Dutch ghevel, Dutch gevel, Old High German gibil, German Giebel, Gothic gibla "gable"). This is traced to a PIE *ghebh-el- "head," which seems to have yielded words meaning both "fork" (such as Old English gafol, geafel, Old Saxon gafala, Dutch gaffel, Old High German gabala "pitchfork," German Gabel "fork;" Old Irish gabul "forked twig") and "head" (such as Old High German gibilla, Old Saxon gibillia "skull"). See cephalo-.
Possibly the primitive meaning of the words may have been 'top', 'vertex'; this may have given rise to the sense of 'gable', and this latter to the sense of 'fork', a gable being originally formed by two pieces of timber crossed at the top supporting the end of the roof-tree. [OED]
Related: Gabled; gables; gable-end.