"temporary passageway" to a ship, building under construction, etc., ultimately from Old English gangweg "road, passage, thoroughfare;" a compound of gang (n.) in its original sense "a going, journey, way, passage" and way (n.). Nautical use dates from 1680s in reference to a passage on the ship, from 1780 of the opening at the side whereby people enter and leave, and by 1840s of the board or bridge they use to get to and from the dock. As a command to clear way, attested by 1912, American English. In British parliamentary use, with somewhat the same sense aisle has in the U.S. Congress.
Below the g[angway], as a parliamentary phrase, is applied to members whose customary seat does not imply close association with the official policy of the party on whose side of the House they sit. [Fowler]