No, it does not mean the "hallway." The "senate gallery" is the balcony, platform or upper floor, projecting from the back or sidewall inside a hall, providing space for an audience to look down into the chamber. You can see good picture of the Texas Senate Gallery (the US Senate Gallery looks similar, but I couldn't find an easily accessible picture on the web). http://www.tspb.state.tx.us/SPB/Gallery/SigSpace/2lg.htm Note that in 1954, Puerto Rican Nationalists shot 30 rounds from semi-automatic pistols from the Ladies' Gallery (a balcony for visitors) of the House of Representatives chamber in the United States Capitol.

The word "gallery" has a lot of meanings. It can be a hallway or it can be a room where paintings and other works of art are shown. But it can also mean the upper part of a theater or the people (the audience) that is in that theater or building. This is the meaning that is used in your example passage. The Congressman gave a speech in the Congress building. Members of Congress sit on chairs on the "floor" of the building. Spectators (the general public) can often sit in the "gallery", the upper part. Those people are also called the "gallery". So the Puerto Rican nationalists fired from the gallery, the upper part of the Congress building, during a speech.