This'll take a decent amount of attempts to get right (even with our guidance), there's a lot of mini-sections to this final part of Chapter 7, so take your time as you'll need a fair bit of patience to see you through!
Now keep the portal at the top of the wall where it is and place another portal directly above it on the slanted portion of the wall. Run over the gel and you'll be bounced into the wall as you hit the puddle of blue Repulsion Gel.
Just before you reach the wall, fire off another portal so you're guaranteed to make it through both portals and onto the upper platform.
As soon as there's enough white gel on ceiling above, get a portal on it and then another one in the blue puddle on the floor below (there's a constant stream of blue gel dripping there). This'll drop blue Repulsion Gel on the floor, so once there's enough there, drop back down to the ground floor.
Walk around to the white gel puddle and set a portal in the middle of it. Now fall through it and drop back down to the ground floor. You'll want to set the second portal on the white gel pool down here. What you're looking to achieve my doing this is coating the wall up above the white gel pool on the upper floor (where you bounced across to).
The main trouble you'll have is that it's only small doses of white gel that's dropped, and it has a tendency for going in a straight line, this makes getting it to touch the walls required much more awkward than it needed to be.
By doing this, you can alternate the position of the gel balls so you have more chance of it coating the upper wall.
Once you're on the walkway, look to your left and you should spot a slanted panel behind a metal grating. Get the exit portal in here so the Conversion Gel is sent into the funnels on the right, coating the whole of the tunnel up ahead.
Don't be deceived about where you end up, you can actually walk straight along this path (even though it looks like a vertical wall). Half way up there should be some white gel laying on the floor, place a portal here then keep waling until you reach the top.