mid-15c., "showy, finely dressed; gay, merry," from Old French galant "courteous," earlier "amusing, entertaining; lively, bold" (14c.), present participle of galer "rejoice, make merry," generally held to be from Latinized verb form of Frankish *wala- "good, well," from Proto-Germanic *wal- (cf. Old High German wallon "to wander, go on a pilgrimage"), from PIE *wel- "to wish, will" (see will (v.)), "but the transition of sense offers difficulties that are not fully cleared up" [OED]. Sense of "politely attentive to women" was adopted 17c. from French.
"man of fashion and pleasure," mid-15c., earlier "dissolute man, rake" (early 15c.); from gallant (adj.).