Gels

Gels are a gameplay implement introduced in Portal 2. They are paint-like liquids that impart different effects on 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 it bounces off. Jumping off such a surface launches the player higher than a normal jump. Objects such as Storage Cubes and Turrets can be covered in Repulsion Gel, which causes them to bounce around erratically.

Propulsion Gel
A bright orange gel that greatly increases object velocity and reduces friction of objects moving on surfaces 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 liquid made from. It enables the player to place portals on surfaces that originally are not portal conductive.

Water
While not techincaly a gel in and of itself, water washes off other gels from surfaces and objects. It has no special properties towards gameplay and does not stick to surfaces.

Trivia

 * Similar to how the student game became Portal, the paint mechanic was taken from a student game called Tag:The Power of Paint.
 * 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.