Once you're past the broken walkway (with the blue gel on it), you'll enter a room with a large number of turrets trained on the exit. You'll never make it by running over the walkway, so look up to your right for a broken window.
Get a portal on the ceiling here, and then set your other portal on the wall directly to your right. You'll fall into a room with a red switch on the floor in front of you. Stand on it and set a portal under the blue gel dispenser (on the ground to your right).