Used as far back as 1998 by Aperture Science, this orange and supposedly sweet and largely non-toxic liquid of fiberglass insulation was originally to serve as a diet aid, serving as a dietetic pudding substitute and marketed under the name "Propulsion Pudding". Its purpose was to increase the velocity of any food following it through the digestive tract, leaving the body no time to absorb calories. However it was pulled from shelves when it was discovered that digestion plays several crucial roles in the eating process, such as breaking food into manageable chunks before being violently expelled from the human body. The Propulsion Pudding was subsequently renamed "Propulsion Gel", and made for use with the ASHPD in Test Chambers instead, while another attempt at a dietetic pudding substitute, the Repulsion Gel, was tested.
- As a testing element, the Propulsion Gel accelerates the velocity of any object it touches, giving them largely increased speed capabilities and allowing them to vault over pits or reach the far end of a long expanse quickly enough to best a ticking clock. Propulsion Gel is spilled through a vent, be dynamic, and play quite well with portals. Placing one portal under a vent and another in different area of a Test Chamber allows the use of the Gel far from the vent, allowing progress past a given obstacle.
- If a Weighted Storage Cube is coated with Propulsion Gel, it will automatically slide on any incline. While this is not used in the single-player campaign, it is an element in the final course of the co-op campaign.