Portal 2 will continue to challenge players to use portals to traverse rooms in unusual manners, such as flinging, in which the player uses the magnitude of the momentum gained by gravitational acceleration through the blue portal to cross a wide gap out of the orange portal.

Portal 2 will continue to challenge the player by solving puzzles in test chambers within the Aperture Science Enrichment Center using the portal gun (the Aperture Science Hand-held Portal Device), a device that can create two portals connecting two surfaces across space. Players, as the silent protagonist Chell from Portal, solve puzzles by using these portals to move unconventionally between rooms or to use the ability to fling objects or themselves across a distance. The functionality of the gun has not changed between the games, but within Portal 2, players can take advantage of the bleeding of other physical effects through the portals. Game Informer identified two examples of this: one was the ability to use air currents created by Pneumatic Diversity Vents, a series of transport pneumatic tubes, through a set of portal openings to push a turret over or to draw objects into the suction. The second example was to use the power of Excursion Funnel tractor beams through portals to bring Chell or other objects to otherwise inaccessible areas. The game also introduces special paint-like gels that can be used to impart certain physical effects to a surface such as Propulsion Gel that boosts Chell's speed as she crosses a surface, and Repulsion Gel that allows her to jump from a surface. The player will be required to determine how to transport that gel to appropriate surfaces using portals in order to progress. The gels can also be applied to objects, such as the Weighted Storage Cube crates, that affect their own physical nature. In addition to the Storage Cube, there are new types of portable objects that assist the player, including Redirection Cubes with mirrored, reflective surfaces used to redirect Thermal Discouragement laser beams, Aerial Faith Plates that can launch objects placed on them, and spherical Weighted Storage Balls, which made a brief appearance in the original game in one of the advanced chambers.



In Portal 2's co-operative mode, two players will control a modified personality core (left) and turret gun, which were evolutions from original Westworld-styled bots.

Portal 2 takes place hundreds of years after the first game. Despite her apparent destruction at the end of Portal, GLaDOS is "still alive". The player controls Chell, the same protagonist from Portal; retroactively patched just prior to the sequel's official announcement, the ending of the first game shows Chell being dragged away by an unseen figure with a robotic voice, where she has been placed in stasis over the years. The game will again take place in the Aperture Science Labs, untouched by human hands but overrun by decay and nature. The player will interact with many of the numerous personality cores (which were seen activating in Portal's conclusion), which have become active and become independent from GLaDOS in the intervening years, using the automated systems of Aperture Science to create their own microcosms within the facility. The cores themselves are unable to move save through overhead rail systems. Chell is awoken by one of these, Wheatley, who has become concerned for the state of decay and seeks to correct it. Wheatley acts as the player's guide during the tutorial and initial stages. Soon, the two encounter the dormant GLaDOS and accidentally wake her; the computer accuses Chell of murdering her years ago. GLaDOS begins to rebuild the ruined facility, and puts Chell under more tests, stating "I think we can put our differences behind us. For science. You monster."
Two new characters will be introduced for the two-player cooperative mode, which will have its own unique plot and setting. These two yet-to-be-named characters include a modified turret gun and a personality core; both units are bipedal and equipped with their own portal guns. Though once part of the networked facility, they have become separate entities and are treated to similar abuse by GLaDOS while Chell is being put through a series of complicated test chambers.



An example test chamber in Portal 2, showing the effect of flora overgrowth and deterioration on the Aperture Science facilities.

Portal 2 was designed to give the player incremental steps in understanding portals and their use within the game. This approach led to two basic types of chambers. The first type, which Valve calls "checklisting", provides a relatively safe environment for the player to experiment with a fundamental aspect of a new gameplay concept. The second type of chamber is one that combines these elements in new ways to make the player think laterally, giving the player a rewarding experience for completing the chamber. Chambers were first developed through whiteboard via isometric drawings, with the developers performing a sanity check on the chamber, before being created into simple levels through the Hammer level editor. Extensive playtesting was used to make sure the solutions to each chamber were neither overtly obvious nor difficult to see, and to observe alternative solutions discovered by playtesters; based on their input, the design team would keep these alternate solutions viable within the level, or would work around and block the alternate solutions if they were too easy. Once a chamber was considered ready to proceed, the Valve artists then would add elements such as detailed texturing, dynamic lighting, and vegetation. These versions would then be sent back for further playtesting to verify the new elements did not prevent players from finding proper solutions, with further iterations between artists and playtesters until such issues were resolved. Johnson stated that Valve's aim was not to make Portal 2 more difficult than the former game, but instead wanted to keep the same idea of a game "where you think your way through particular parts of the level, and feel really smart when you solve it". While the single player campaign in Portal 2 is designed to avoid frustrating the player, the co-operative levels are more focused on coordination and communication, and are recognized by Valve as being much more difficult than the single-player puzzles.



Portal 2 introduces paint-like gels that affect the physical nature of surfaces they coat. Here, the blue Repulsion Gel, carried over the turret guns by the faint blue Excursion Funnel beam, causes the painted turrets to bounce off any other surface.

Portal 2's gameplay includes a gel mechanic that can alter the physics of surfaces coated with that gel. Valve found that this addition gave players more control over the game world, but as a result, required the chamber designer to be more devious with their solutions to account for the various possibilities of the gel mechanics. The gel mechanic comes from Tag: The Power of Paint, a DigiPen student-developed game that won the 2009 Independent Games Festival Student Competition prize. Tag Team, a group of the students that worked on Tag, was hired by Valve. Journalists compared this to the evolution of Narbacular Drop, another DigiPen student project, into the basis of Portal. Portal 2 also contains advanced rendering techniques for liquids that were developed from Left 4 Dead 2. Portal 2 combines the concepts of "flowing" surface maps to mimic the motion of water in a setting, along with "debris flow" maps and random noise to create realistic real-time rendering of water effects.