To look inside G-Man's briefcase, first enable the noclip code. There are two ways to do this trick: the first is to begin from the start and ride the train until you see the G Man for the first time. He should be in a train next to the one you are in towards the end of the ride, by the radioactive water. The other way is to go through the train ride, then walk around the halls of the laboratory to find him talking to a scientist. When you do this, go towards the G-Man. Get as close as possible to his briefcase without going through it (this make require several attempts). In one side, there is paper, three pencils and a pistol. On the other side is a computer with a message and a picture of the G-Man. Note: When doing this where the G-Man is talking to the scientist, you will not be able to understand what the two are saying.