He was on Half Life 1, I noticed also that you see him throughout the game on Half Life 1, talking to people and stuff, you also see him on the expansions, like blue-shift and opposing force...
On Half Life 2 though, I noticed him in lots of places, throughout the game, the vortigons (Aliens), are watching it, and if you get in fast enough, you can see the G-Man on the Television set. You also see him in high places, watching you from above, then walk off with his briefcase, I have found some pictures on the internet. There is a few that they haven't noted, and that is the one with the alien watching on the television, and the one where you are in a room, a tv is on it's side
without the plug in, and the gman is on it... (On this game, things are exact, a tv doesn't work without the plug socket etc.
-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.
There are more, they're the ones I can only find.