To the left you see a sign that says Gamma. This is Pump Station Gamma. Go straight ahead through the emancipation grill. In this lobby, there is another disabled elevator to the surface. Nearby, behind some chairs, there is a white wall. Put a portal there, then turn around and put a portal of the other color on the wall up above near another emancipation grid. Go through the grid toward the "pump station gamma" sign.
Look in the direction the sign is pointing in, and look up. There is a white wall up there where it says Gamma in big letters. Put a portal up there and put another one nearby. Go through. There's a door in the wall near you. Go in and go through the fizzler grid.
Turn left and go along the walkway along the left wall. You'll reach an office with three levers. Pull all three to activate the blue, orange, and white gel. This also activates some pistons, so don't exit the office too quickly. The pipes have some openings in them, so a bunch of gel goes all over the place.
Look up above the pipes. The white gel coated a ceiling over a metal ledge. Put a portal on the white gel. Maybe you have guessed what white gel does: you can put portals onto a surface that has been coated with white gel (but only if that surface is flat, and large enough for a portal). After putting a portal on the white gel above the pipes, go into the office and make a portal of the other color on a wall in here. Go through.
Now you are on a walkway up above the pipes. All you need to do is go forward and fall off. There is blue gel down there that you will bounce from, and you will end up on the ledge where it says "keep out".
Go left and go through the fizzler. Now you have to carefully run along the orange gel past the pistons before they squash you. Wait for the pistons to start rising before you run. If you bump into the first piston, go back and try again, waiting a little longer before you start running.
After you make it past the pistons, turn right and push the exit door open. As you can see, there is a ledge pretty far away that you need to reach. It's across from the door that you just came out of. First, make a portal on the angled wall next to the door that you came from. Then go back to the piston room. Put a portal of the other color on the wall past the pistons. Now run along the orange gel, through the portal, and you should be thrown over to the distant ledge.
Go up the stairs and through the computer area. Drop down through the broken window to the right, down to where there is a Cave Johnson portrait. Go through the door to the right of the portrait and go to the office at the end where there is a red button. Press it.
Go back out and go through the door to the left of the Cave Johnson portrait. Go through the door that the red button opened. Now go along the walkway to the elevator and go in.