i found this, it tells all the places where u could see G-man
Chapter 1: At the beginning.
Chapter Two - If you look at the monitor that Barney
activates when he first enters
Dr. Kliener's lab, you can change the
channel to get a good look at the
G-Man for a few brief seconds. Once
he realizes he's being watched, he
walks away. Hrm...
Chapter Three - In the first box car you come
to where the Vortigaunt is
powering the television, you can
see the G-Man smiling at you for a few
brief seconds on the television.
(Use your suit's Zoom)
Chapter Four - Standing on the Barn that is
way station 12. You can only
see him from a distance before
he runs away and does his
disappearing act.
Chapter Four - In one of the areas where you go
beneath a bridge, the G-Man can
be seen on a catwalk on the upper
left of the bridge, before you
pass beneath it.
Chapter Four - At the very end of the chapter,
after you have defeated the
Hunter Helecopter, you can
see the G-Man walking along the
dam towards the switch you need
to pull to open your path down to
Black Mesa East.
Chapter Six - At the very end of this level when
you are on the train tracks.
If you go up to a pair of train
cars that are wedged in a V shape,
and look through the crack on the
left of them by crouching, you can see
the G-Man walking away.
Chapter Seven - Through binoculars in Beach House
#2. It's interesting to note
that in this sighting he appears
to be having a conversation with
another person.
*Thank you to numerous people who pointed
out that the person he is talking to is
in fact the Colonel who gives you the RPG
launcher. Thank you guys.
Chapter Nine - In the room with the first two
chargers that you see. If you
change the video screen in there
to the third "Channel" you can see
the G-Man for a few seconds.
Chapter Eleven - On a television in the first
building after you get separated
from Dog. It is a brief image of
him before the television goes
dead. Plus, if you check, it was
never plugged in. Creepy.
Chapter 14: At the end.
