The gangway is the means of getting on and off a ship. In general shipping terms, it refers to a walkway or bridge connecting the vessel to land. Because access varies widely from one port to another, a cruise ship gangway simply refers to the place on the ship where you enter and exit.
On embarkation and debarkation days, for example, the gangway will often consist of an enclosed raised bridge like a jetway in an airport. That gangway connects a main deck of the ship directly to the cruise terminal. At ports of call where the ship is parked at a dock, the gangway is usually on a lower deck, where a metal walkway bridges the gap between the ship and the dock. At ports where the ship does not dock and passengers are taken to shore in tender boats, the gangway will again be located on a lower deck where passengers are helped across a walkway or platform to the tender boat.
Published notices, announcements and signs should point you to the proper deck and location of the gangway at each port of call during your cruise.