from Old English gang "a going, journey, way, passage," and Old Norse gangr "a group of men, a set," both from Proto-Germanic *gangaz (source also of Old Saxon, Old Frisian, Danish, Dutch, Old High German, German gang, Old Norse gangr, Gothic gagg "act of going"), of uncertain origin, perhaps from PIE root *ghengh- "to step" (source also of Sanskrit jangha "shank," Avestan zanga- "ankle," Lithuanian žengiu "I stride"). Not considered to be related to go.

The sense evolution is probably via meaning "a set of articles that usually are taken together in going" (mid-14c.), especially a set of tools used on the same job. By 1620s this had been extended in nautical speech to mean "a company of workmen," and by 1630s the word was being used, with disapproving overtones, for "any band of persons traveling together," then "a criminal gang or company" (gang of thieves, gang of roughs, etc.). By 1855 gang was being used in the sense "group of criminal or mischievous boys in a city." In American English, especially of slaves working on plantations (1724). Also formerly used of animal herds or flocks (17c.-19c.). Gangway preserves the original sense of the word, as does gang-plank.

1856, from gang (n.). Related: Ganged; ganging. To gang up (on) is attested by 1919.

from Sanskrit ganga "current, river."

"detachment under command of an officer empowered to press men into public service," 1690s, from press (v.2) + gang (n.).

"a number of slaves or convicts chained together outdoors doing labor or during transit," 1816, from chain (n.) + gang (n.). 

militant Zionist terrorist organization (officially Lohame Herut Yisra'el "Fighters for the Freedom of Israel") founded 1940 by Avram Stern (1907-1942).

1976, translating Chinese sirenbang, the nickname given to the four leaders of the Cultural Revolution who took the fall in Communist China after the death of Mao.

"a moving" in any way, c. 1300, verbal noun from go (v.). The Old English verbal noun was gang "a going, journey; passage, course" (see gang (n.)). Meaning "condition of a road or route for travel" is from 1848, American English; hence to go while the going is good (1907). Going to "be about to" is from late 15c. Goings-on "(questionable) proceedings" attested from 1775.

by 2000 as the verb that indicates the situation of many players or NPCs simultaneously attacking one; gamer slang, perhaps borrowed from hip-hop and drug-abuse slang (where it is attested by 1995 in the sense of "to rob, to rip off"); perhaps by 1990 in sports jargon. Of unknown origin; perhaps ultimately based on gang (v.). Related: Ganked; ganking.

1953, "group sex" (especially many men on one woman or girl, regardless of consent), from gang + bang (v.) in its slang, "perform sexual intercourse" sense. Earlier was gang-shag (1927). Sense of "participate in a street gang" is by 1968. Related: Gang-banger; gang-banging.