after being tricked to push a button which just explodes, you have to lay a portal under the boss. The explosion opens up a hole in the roof of the facility and reveals a full moon... You fire at the moon, and everything gets sucked in by gravity, but GlaDOS saves you because she's awesome.

That's not how gravity works. That's decompression. Also you got the order of events all mixed up but that's not very important really.

GLaDOS is amazing.

running on the propulsion gel into a portal is the same as falling into it from a long drop. Also, pay attention to the arrows on the floor. also also, you put the gel on the panel BEFORE you turn it the right way up.

Coat the long runway with the red gel. If you stand on that runway and look in the opposite direction of that floating island where you found the cube, You will see a ramp you can portal too. Coat the ramp in the blue gel. (i will call this ramp ''Ramp A'' so i can refer back to it later.

Then portal over to the place with the switch and the platform that flips when you step on said switch. While you AREN'T on the button, and the platform is still upside down, create a portal underneath it and use it to coat the platform with blue gel.

THEN, place the the cube on the switch so the platform turns the right way up and portal back to the runway.

Nearly there now. Place one portal at the end of the runway, and the other on the wall facing 'Ramp A'

Run down the runway into the portal building up speed. You'll bounce over and land on the flipping platform you put blue gel on. You'll still have all that momentum so you'll bounce again and land on the platform that leads to the exit.

I realise you've probably already figured it out by now.