-Chapter 1: At the beginning.

-Chapter 2: On a monitor screen in Dr. Kleiner's lab after Barney finishes at the terminal.

-Chapter 3: On the TV that the Vortigaunt is watching inside the red train carriage.

-Chapter 4: Standing on the pier in front of Station 7 at the beginning of the chapter.

-Chapter 4: Flickering on a giant monitor attached to the high-rise building. It is very quick, but if you look carefully you can see the skeletal form of G-Man on the screen.

-Chapter 6: On a small gantry near the second covered bridge.

-Chapter 6: In the last part of the chapter you will come across a pier next to a dam. Get onto the pier by driving up the ramp in your speedboat and turn left by the big red warehouse. You can see G-Man staring to the right at the bottom of the rest of the pier.

-Walking into a train tunnel beyond the railcar blocking the railroad. See the "Hidden G-Man" hint for a detailed description. Note: This sighting is not in the official strategy guide.

-Chapter 9: Looking through a window near a pair of double doors on a monitor screen at the beginning.

-Chapter 11: On a TV at the ruined foyer at the tenement block after you and Dog get separated.

-Chapter 14: At the end.