Walk to the end of the catwalk, then drop down to the area below. Go forward and look for some metal to the right that goes over to another platform. Walk along that metal to get there.
Go to the far corner of this area and turn around. You should see a white wall very high up on a column. Make a portal on it, then make a portal of the other color near you. Go through.
Turn left and you'll see a testing area. Make a portal on the floor there, then make a portal of the other color near you and go through.
Press the red button to turn on the flow of blue gel. Put a portal on the spot that the gel is falling on. Then go over to the other side of the room where there are some ledges. Make a portal of the other color on a wall close to the ledge. The repulsion gel will spread on the floor. Use it to bounce up to the next ledge. Do the same thing to go up to the next ledge. (So much for not getting covered in repulsion gel.)
Now turn around. There is a big emancipation grid with an angled white wall piece behind it. There is a ledge below that. PPut some gel on the floor between you and that ledge so you can jump up to there. Then put a portal on the floor below you, then move that portal elsewhere before the gel goes back through. This will put some gel on the floor just below the emancipation grill.
Bounce through the emancipation grid. This will get rid of the portals that you created. Put a portal on the angled wall. Then go around the ledge, and without jumping down, put a portal of the other color on the place that the gel is falling on. This puts a blob of gel on the ledge to the right that has two orange arrows on it pointing toward the exit.
Now put a portal on the floor below you. Make sure that a portal of the other color is on the angled wall. Drop into the floor portal. You should bounce off of the gel and reach the exit. Follow the path to the elevator.