This guide will detail every sighting of the G-Man in Half-Life.

These screenshots were taken from Half-Life Source (and some gmod) for the easy level select, but there is no difference with his sightings between the two versions.

This guide contains spoilers.

G-Man can be seen in the game’s opening; he is on a tram to the left of Gordon’s in the room with the nuclear waste, just before reaching the end of the line.

Before getting the HEV suit, head to the furthest exit on the left once you leave the lobby. G-Man will be arguing with a scientist in a nearby room.

In the room with some houndeyes (where you find a scientist hiding you can take to open a supply room). G-Man is standing on a catwalk when you first enter, and will leave if you venture too far into the room.

Just before the hallway with the jump to the elevator, go to your right and look through the door’s window. G-Man will appear and look at Freeman for a few seconds before leaving.

On a catwalk just after the part with the turrets, water and scientists (that like to get shot by said turrets), and just before dealing with the trip-mine. He will leave if you go too far into the room.

At the beginning of the chapter, look up into the control room before you make a run for it. G-Man will be watching Gordon.

In the room with the moving platforms, while still in the water, look above to where you eventually must go. He will leave quite quickly after exiting the water.

When reaching the portal segment of this chapter, look through a window after coming across the security guard. G-Man will walk through a portal.

Obviously, he appears at the end. I suggest employment.

As Uplink is playable now; I thought I'd add this G-Man sighting. When in the room with the Gargantua, G-Man will be watching the scene and then calmly walk away as the windows smash. It's actually a very cool sighting in my opinion.
At the moment I only have this screenshot from a different angle and taken in Sven Co Op; I will get a better screenshot of it at a later time.

Opposing Force

Blue Shift

Black Mesa

Half-Life 2

Half-Life 2: Episode One

Half-Life 2: Episode Two