Which DigiPen game inspired Portal 2's Repulsion and Propulsion gels?

The DigiPen game that inspired Portal 2’s Repulsion and Propulsion gels is Tag: The Power of Paint. Valve incorporated the student project’s paint-based movement ideas into Portal 2’s puzzle design.

Tag was created by a team of DigiPen Institute of Technology students and released in 2008. Its central mechanic involved spraying different paints onto surfaces to change how the player moved across them. The effects included bouncing, gaining momentum, and walking across unusual surfaces.

Portal 2 adapts that concept into gels rather than a handheld paint gun. Repulsion Gel makes the player or objects bounce, while Propulsion Gel increases movement speed. A third gel, Conversion Gel, lets portal surfaces function on otherwise unsuitable materials and was not the same movement effect as the two gels named in the question.

Tag’s connection is sometimes confused with Narbacular Drop, another student game associated with Portal. Narbacular Drop helped inspire Portal’s portal mechanic and led to its developers joining Valve; Tag specifically supplied the paint-based inspiration for Portal 2.