Repulsion Gel
Repulsion Gel is a light blue substance that repels objects. A player or an object that hits a surface coated with the Gel bounces off. Due to the effect, the height from which a player falls before landing on a Gel-covered surface affects the height of a jump. Players can also use the Gel to move back and forth between two parallel surface rather than merely be propelled up and down. Objects such as Storage Cubes and Turrets can be covered in Repulsion Gel which causes them to bounce around erratically.
Propulsion Gel
Propulsion Gel is an orange substance that greatly increases object velocity and reduces the friction of objects moving on the surface(s) it is applied to. Objects covered in Propulsion Gel similarly have their friction reduced, causing them to slide around.
Conversion Gel
Conversion Gel is a portal-conducting white substance made from ground moon rocks. It enables the player to place portals on surfaces that are not normally portal-conductive.
Unused Adhesion Gel
The purple "sticky" or "adhesion" gel was found on accident in the Portal 2 authoring tools. It turns out that the purple gel was initially going to behave like the blue stick paint in Tag: The Power of Paint, the game off of which Portal 2's gel mechanics are based. The adhesion gel would have been in the game along with the repulsion, propulsion, and conversion gels in Portal 2 now. But some play testers got too disoriented when they had to think with portals while walking on walls and ceilings, so Valve ended up removing the gel before it was completed.
Unused Reflection Gel
The Reflection Gel replaced the Adhesion Gel in the authoring tools after Portal 2 major update of their first DLC. The gel uses the Conversion Gel texture when hits the floors, and causes the Thermal Discouragement Beam to reflect, similar to the Discouragement Redirection Cube. When this gel hits the player it shows the screen effect of the Propulsion Gel.
Water
While not technically a gel, water washes off other gels from surfaces and objects. It has no special properties in regards to gameplay, and does not stick to surfaces.
Slime
Slime or Sewage was found only in Portal during Chell's escape, and is not often considered a gel due to the fact it has no emitter, is not used to solve chambers, and the player cannot "paint" with it like with the official gels introduced in Portal 2. Regardless, it does have some properties similar to some gels in Portal 2, such as the slippery effect of running on Propulsion gel and an adhesion effect that keeps the player stuck on the ground.
