"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]