"long and loose-jointed," by 1812, from Scottish and Northern English gang (v.) "to walk, go," which is a survival of Old English gangan, which is related to gang (n.). The form of the word is that of a present-participle adjective from a frequentative verb (as in fondling, trampling), but no intermediate forms are known. The sense extension would seem to be via some notion involving looseness in walking.

GANGLING. Tall, slender, delicate, generally applied to plants. Warw. [James O. Halliwell, "A Dictionary of Archaic and Provincial Words," 1846]

"period between youth and old age," formerly generally understood as 40 to 50, late 14c., from middle (adj.) + age (n.). The adjective middle-aged "having lived to the middle of the ordinary human lifespan, neither old nor young" is by c. 1600.

"habitually attending church," 1540s, from the verbal phrase; go to church for "attend divine service in a religious building" is from late 12c. Late Old English had church-gang for "attendance at church." Related: Church-goer.

"apparition of a living person, 1826 (from 1824 as a German word in English), from German Doppelgänger, literally "double-goer," originally with a ghostly sense. See double + gang (n.). Sometimes half-Englished as doubleganger.