Galleria and Passaggio can both mean several diffrent things, but a Galleria is always covered. For example a covered piece of Street, or a tunnel. Passaggio instead implies a way THROUGH something: life for example a way across buildings or a pedestrian crossing. Forestiero and Straniero are basically the same thing, but Forestiero is kind of old italian, and it's not quite used anymore. People would usually say Straniero.