Codename: Gordon is a 2D, Adobe Flash-based side-scrolling shoot 'em up developed by Nuclearvision and published by Valve Corporation, as a promotion for Half-Life 2. Despite being heavily based around Half-Life 2, Codename: Gordon was released in May 2004, roughly six months prior to Half-Life 2's eventual November 2004 release. Due to this, Codename: Gordon was the first in-game appearance of many aspects of Half-Life 2, as well as some designs and features which were scrapped during Half-Life 2's development.
In Codename: Gordon, players undertake the role of Gordon Freeman, the silent protagonist of the Half-Life series, as he travels through various locations inspired by the Half-Life universe, including City 17 itself. The game features several recognizable enemies from Half-Life including headcrabs, zombies, and the Combine, as well as some of the series' characters, such as Alyx Vance and Barney Calhoun. The game features four weapons, all based on weapons from Half-Life 2. The game also features dialogue options, however only for comedic purpose, as all response options are some variation of silence.
Finishing the game unlocks "Crow Chase", a bonus mode in which the player has to chase crows, earning points the longer the crows remain air-bound.
Codename: Gordon was distributed for free, exclusively through Steam. The game was hidden on Steam following the bankruptcy of Nuclearvision, due to issues caused by hard-coded calls to their now-inaccessible website. However, the game can still be installed and played by putting the custom steam URL "steam://install/92" into an address bar, or the Steam command console.
Codename: Gordon is set in the universe of Half-Life 2, with Gordon Freeman encountering other Half-Life characters throughout various locations themed after the game. The characters regularly remark on having become two-dimensional, with the G-Man eventually instructing Gordon Freeman to kill a Combine Strider in order to learn why the world had become two-dimensional. Upon defeating the Combine Strider, the game's credits play.