This is just a quick video for a tip on part of the gnome achievement ("Little Rocket Man"). Since you aren't able to get back into this room, I propped a crate on a small health pack, and put the gnome on top. After I went all the way around and got the car, I was able to hit the switch to open the door, and have him fall through (with a little push from the Vortigaunt). Otherwise you'd have to be really quick with the gravity gun.
Sorry for the quality, I was being lazy and just filmed the tv screen (xbox 360 version).