Gels

Gels are a gameplay mechanic introduced in Portal 2. They are paint-like liquids that impart different effects when applied to surfaces and objects.

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. It enables the player to place portals on surfaces that are not normally portal-conductive.

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.

Trivia

 * A fourth gel, "Sticky Gel", was tested within Portal 2, however it was removed early on during the development due to making play-testers motion-sick.
 * Repulsion Gel was Aperture Science's first attempt at creating a dietetic pudding substitute.
 * Similar to the way in which the student game became Portal, the paint mechanic was borrowed from a student game called Tag: The Power of Paint.