Gel (Portal 2)
Gel or paint is a puzzle element of Portal 2. It flows out of droppers in blobs, and covers any surface or object it splashes on, changing its physical properties. Water streams behave the same way gel does, but cleans gel off surfaces. Gel streams can be redirected with portals and funnels.
There are three types of gel, and water. Repulsion gel is blue, and causes things to bounce around. Propulsion gel is orange, and makes objects slippery and players move quickly. Conversion gel is white, and allows the placement of portals.
Creating a Gel Dropper
1. Create a func_instance entity with the following properties:
Property Name Value Fix up Name gel_dropper VMF Filename instances/gameplay/paint_dropper.vmf $trigger_to_start start_gel $trigger_to_stop stop_gel $paint_type 0 = Bounce, 1 = Stick, 2 = Speed, 3 = Conversion, 4 = Water
If the cube dropper doesn't show up then save and reload so you can position it properly.
2. Create the entity you want to activate the gel dropper, such as a trigger_once at the entrance to the room, and give it the following outputs:
My Output Target Entity Target Input Parameter Delay Only Once 
OnTrigger gel_dropper instance:@start_gel;Trigger 0.00 No
3. Hammer may indicate that this is invalid. Ignore it.