ok this is what I'd do, listening? good.

Idea :1) make a big room or long hall what ever is like your level and put the open ele door at the end, then put a trigger once right on the other side of the door so when the G-man walks through the door the door closes quickly before the player can pass too. (note: you can use this practice room to easily tweak it ontell you get the distance right so the the player cant get through in time.

Idea 2 If you didnt want to have a door and jusy have a invisable force field that make a brush as big as the hall/path and put the invis texture on all the faces ( i assume you know the propertys to make things invisible) and make it a func_door, and since its invisable you can set the move speed to something like 200. Its pritty much the same thing you just have an invisable door instead.

hope this helps.

then you make the g-man close to the door to begin with and the player far away. even though the g-man is slower then the player, if you put the g-man close enough and the player far away enough it wont be posible for him to get there in time.

I dont know how your map is layed out but you could put it in a hall that has a (L) shape and have the payer come from the bottom and see the g-man just as hes about to enter the elevater heck you have the player player 20 feet away and just pop around the corner to see the g- man just enter the elevater ant the doors close behin him. beleave me if you want to have the g-man be all misterius like that then its not how long the player sees the man its just getting a fast glimps of him that realy strange and makes the player wonder.

If you are still unsure of how i mean about timing it right and still thing it can be done look at the beginning of the mod Redemption downloaded here

>>>>>http://www.planethalflife.com/espacemod/us_index.html

it has a scientist do the same thing your talking about.