"long, loose outer garment," 1510s, from Spanish gabardina, which Watkins says is from French galverdine, from a Germanic source such as Middle High German wallevart "pilgrimage" (German Wallfahrt) in the sense of "pilgrim's cloak." The compound would represent Proto-Germanic *wal- (source also of Old High German wallon "to roam, wander, go on a pilgrimage;" see gallant (adj.)) and Proto-Germanic *faran "to go" (from PIE root *per- (2) "to lead, pass over"). The Spanish form perhaps was influenced by Spanish gabán "overcoat" and tabardina "coarse coat." Century Dictionary, however, says the Spanish word is an extended form of gabán and the Spanish word was borrowed and underwent alterations in Old French.
"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.