01 - "Injection Point" - d1_trainstation_01
G-Man appears at the very beginning of the game and brings Gordon out of stasis.
02 - "Red Letter Day" - d1_trainstation_05
After Barney arrives at Kleiner's lab, he'll turn on the camera monitor. Walk up to him and cycle through them until you see the G-Man standing by the chain-link fence. After a while, he'll straighten his tie and leave.
03 - "Through the Channels" - d1_canals_01
Continue through the story. After a while, you'll need to go through a red container where a Vortigaunt and a Rebel are hiding. Pay attention to the TV. For a while, you can see the G-Man in the place of the now familiar Dr. Breen.
04 - "Dangerous Water" - d1_canals_06
G-Man can be spotted on the dock of the red barn. As you approach, he will go inside and disappear. If you attempt to follow him, you will be met by two zombies and a headcrab.
05 - "Dangerous Water" - d1_canals_06
One of the most challenging stashes.
Continue through the story. After a while, you'll come across a broken monitor attached to a high-rise building. The monitor is broadcasting Breen's speech, but the G-Man can be seen in the static.
06 - "Dangerous Water" - d1_canals_12
G-Man stands on a metal bridge. When approached, he straightens his tie and disappears through the door. Unfortunately, following him is impossible.
07 - "Dangerous Water" - d1_canals_13
At the very end of the chapter, after the battle with the helicopter, head up to the red hangar. Look around the corner – the G-Man will be standing near the dam, watching your movements closely. After a while, as always, he'll straighten his tie and disappear around the nearest corner. If you try to follow him, you won't find him.
08 - "We don't go to Ravenholm..." - d1_town_05
At the very end of the chapter, in front of the hangar with the rebels, you'll see the tracks blocked by broken train cars. If you look under the leftmost car, you can see the G-Man quickly disappearing from his observation point.
09 - "Route 17" - d2_coast_03
While traveling along the coast, you'll come across a white house with a red hangar. Enter and kill all the Alliance soldiers. Attached to one of the windows will be binoculars, through which a patrol was observing the rebel camp. If you look through these same binoculars, you can see the G-Man talking to Colonel Cubbage on the building's balcony.
10 - "Nova Prospekt" - d2_prison_02
At the beginning of the chapter, you'll find a small security post with a monitor broadcasting surveillance footage. Cycle through the cameras until you see a small room with a door. After a while, the G-Man will approach the window, straighten his tie, and disappear.
11 - "Trespasser Number One" - d3_c17_02
After you and Dog clear the streets of City 17, head toward the metal doors. As soon as they close, music will start playing. Follow the sound into the building's entrance. In the back room, littered with rocks, you'll see a TV showing the G-Man, with a crow on his shoulder, adjusting his tie. The TV will immediately turn off after that.
12 - "Intruder Number One" - d3_c17_03
In the station square, you can see rebels attempting to knock down the screen broadcasting Dr. Breen's broadcast. If you look closely, you can briefly make out the G-Man's silhouette in the static.
13 - "Dark Energy" - d3_breen_01
At the very end of the game, after the Combine teleporter explodes, the G-Man stops time and talks to Gordon.
(Bonus) - "Black Mesa East" - d1_eli_01
Concept artist Dabih Eng hid an image of G-Man in the textures. In the "Black Mesa East" chapter, after you descend into Eli's lab, approach the Vortigaunt's workbench. He'll be working on a device nearby. Open the console, enter "sv_cheats 1" and "noclip," and then fly inside the device.
You'll see a bright yellow texture. Use third-party programs to adjust the contrast.
Author: cupcake