Used as far back as 1953 by Aperture Science, the Repulsion Gel is the company's first attempt at a dietetic pudding substitute. The gel, a bright blue color, is said to be a sweeter, slightly less non-toxic form of fiberglass insulation. Its purpose was to cause subsequently ingested food items to bounce off the lining of the dieter's distended stomach and out of their mouth. However, for various reasons, the product was also pulled from the shelves like the Propulsion Gel, and reconverted into a testing element for use with the Aperture Science Portable Quantum Tunneling Device, the forerunner of the Aperture Science Handheld Portal Device. The material was eventually deemed hazardous even by Aperture Science's standards, and sealed away in Test Shaft 9 along with the Propulsion Gel and Conversion Gel, before the shaft was opened by Chell in order to escape from it. During the escape, she activated the pump stations containing the gel, and finally opened the main hatch between Test Shaft 9 and the main facility, allowing the Gels to once more become available in the facility.
As a testing element, the Repulsion Gel, coming as single blobs or continuous streams through a vent, transfers its properties to any object it touches, giving them a trampoline-like surface. After coating parallel walls on either side of a large pit, a Test Subject ricochets back and forth across the gap. Plastering Repulsion Gel all over a Weighted Storage Cube causes it to barrel across the room, shattering glass and taking out Sentry Turrets in its path. Portals can also be shot under Repulsion Gel vents, allowing the gel to be sent in any area of a Test Chamber.