Gex is a 1995 platform game developed and published by Crystal Dynamics for the 3DO. It was followed by ports for the PlayStation and Sega Saturn developed by Beam Software, and a Windows version published by Microsoft. It was a pack-in game for Panasonic models of the 3DO later in the console's life. It is the first in the Gex series of video games, and introduces players to the title character, a wisecracking, television-obsessed gecko voiced by comedian Dana Gould, who must venture through the "Media Dimension" and defeat Rez, the overlord of the dimension who wants to make Gex into his new network mascot.

Gex was created by Lyle Hall in 1993 shortly after he had joined Crystal Dynamics, and initially followed a movie stuntman named Gecko X before being retooled at the advisory of lead programmer Gregg Tavares. The game's lead character was intended as a mascot for the developer who could rival the likes of other immensely popular platformer characters – primarily Mario and Sonic the Hedgehog. Initially intended for completion in June 1994, and later in September of that year, development of the game took 21 months, with time constraints and a limited development team leading to numerous production difficulties. Several features were cut because of a necessity to complete the game on time, but some of these features were later re-added by a team of developers who programmed several other secret features into the game.

It was released in April 1995 to largely positive reviews from critics; particular praise was aimed at Gex's unique platforming abilities and Gould's voice work as the character. The 3DO version of the game managed to sell over a million copies, making it one of the system's better-selling games. Two sequels were later released, titled Gex: Enter the Gecko and Gex 3: Deep Cover Gecko, with Gex becoming Crystal Dynamics' mascot for a short time.

Gex is a side-scrolling platformer that follows the title character Gex, an anthropomorphic, television-obsessed gecko who must travel through the "media dimension" and defeat the game's antagonist, Rez. Gex must traverse through 24 levels contained in five different TV channels which act as game worlds (accessed through a world map), each filled with platforming stages and ending with a boss stage. The goal of each stage is to explore and locate hidden television remotes which are used to unlock more levels. The original 3DO version of the game allows players to save their progress and resume from where they left off; all other versions use a password system. The player unlocks passwords or is granted a save opportunity by beating a boss stage or finding video tapes hidden in certain levels. Every level contains a hidden portal that leads to a bonus level, the perfect completion of which earns the player a piece of the Planet X remote. Collecting every piece of this remote allows them to reach Planet X, an optional secret world.

Many of Gex's default abilities take advantage of his special characteristics as a gecko. In addition to being able to walk, run, and jump, he can attach himself to walls and ceilings and crawl along them using the suction pads on his feet, allowing him to reach higher areas. Gex primarily attacks with his tail; he can whip it in a full circle to defeat enemies, activate switches, and deflect projectiles. He can coil it into the shape of spring to bounce off enemies and other objects. Levels contain assorted collectible power-ups which can provide Gex with several different abilities, such as enhanced speed, invulnerability, and one of many different types of projectiles that can be launched from his mouth. Gex can obtain and utilize these power-ups by lashing his gecko tongue out and consuming them; alternatively, these items can be tail whipped to replenish lost health at the expense of the item's primary effect. Gex's health is represented by three "hit paws", and he loses one whenever he is hit by an enemy or a hazard. If the player loses all of their hit paws, then they lose a life. The number of hit paws can be extended to six by finding a certain power-up. The player begins the game with three lives, though more can be earned by collecting a hundred Golden flies in a level. Losing all lives causes the game to be over, forcing players to restart from the beginning (or from the last saved in the 3DO release).

Gex, a young anthropomorphic gecko, lives by himself watching television and eating snacks in his mansion in Maui, Hawaii, which he acquired after inheriting a large sum of money following the death of his great-uncle. While looking for a good show to watch after doing some "nude funkercising", he consumes a passing housefly. This house fly turns out to be a small undercover drone being controlled by Rez, the overlord of the Media Dimension. Rez uses the droid to "bug" Gex, and pulls him into the Media Dimension through the TV set, intending to use him as the network's new mascot character. To escape, Gex needs to traverse the Media Dimension and find remote controls that he could use to destroy the TV sets blocking his exit back to the outside world.

Gex fights his way through the Media Dimension, finding remotes, and defeating Rez's henchmen along the way, before eventually defeating Rez himself using some of the tyrant's drones against him. Upon returning home, Gex resumes watching his TV, wondering what was on HBO.

The concept for Gex was created by Lyle Hall, who began work on the project shortly after joining the newly-formed Crystal Dynamics in 1993. Hall wanted the game to "take advantage of both the graphics prowess and the CD audio capabilities" of Panasonic's 3DO console, intending to create a 2D platform game starring "the coolest character I could come up with". It was created with the intention of a new mascot in mind, with Crystal Dynamics pushing for a character who could rival the likes of other gaming icons such as Mario and Sonic the Hedgehog. Initially, the development team consisted of four people; Hall, who served as the game's producer, Mira Ross and Susanne Dougherty as artists, and Gregg Travers as the lead programmer (later on, Justin Knorr was hired as the game's lead designer). The game initially centered around a Hollywood film stunt performer named "Gecko X" who needed to help save his contracted film studio from going bankrupt at the volition of the antagonist, Karl Chameleon. Each stage was themed around a film genre (e.g. a level inspired by Western films) and would begin by showing stock footage from a vintage film of that genre. The player would then traverse the level and perform "stunts" along the way, with the player's performance dictating the amount of money the film would earn at the box office and how well the studio would do. The concept was eventually nixed at the suggestion of Travers, who argued that placing the levels in such realistic settings would lead to a lack of sensible design structure, and a new, more fantastical concept was brainstormed, in which a new villain, named Rezull, would capture Gex and bring him into a "TV Land" where he would have to escape by collecting magical TV remotes and fighting Rezull's armada of "video warriors" made of TV static. Gex's voice was provided by comedian Dana Gould, who wrote all of the character's dialogue.