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 and coloring it if blue or orange. Water streams behave the same way gel does, but cleans gel off surfaces. Gel streams can be redirected with portals and funnels.
Note:For gel to render properly, you must have at least one light entity.There are three main types of gel. 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.
Note:The stick gel does not do what it is suppose to do anymore. It was taken out of the game. However if you enter in stick gel value you will get a really white gel (whiter than conversion gel) that when it covers a surface it looks like conversion gel. This gel, however, now reflects lasers to power laser catchers and relays. If it splashes on you or covers objects(besides cubes) it will look like speed gel. This gel also will stop a speed gel covered cube or object. If you use this gel with a Prop_paint_bomb then when it explodes it will explode purple. These last two things are leftover parts of the programming.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 $paint_sprayer gel_sprayer $trigger_to_start start $trigger_to_stop stop $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-start Trigger 0.00 No
Note:Hammer will indicate that this is invalid. Ignore it.Notes
1. For paint to function in your map you need to turn on the flag "Paint in map" in the map properties.
2. For paint to stick to surfaces you NEED to run RAD on Fast or Normal, otherwise it will only coat certain entities like turrets and cubes.
3. Gels won't appear if the maps are leaked.