The gameplay is very different from most third-person shooters, due to a unique dual analog control scheme, which is the main contributor to the steep learning curve of the game. Players travel to various worlds investigating the disappearance of several colonies, with only giant mutated insect creatures left behind. These insects comprise the bulk of the enemies you face along the way.
Players control Kelly or Saburouta with both analog sticks, the left controlling forward/backward movement and turning, while the right aims their weapons, and when clicked, allows quick-turns. Clicking in the left stick while pointing in a direction causes the character to boost. The left trigger is a boost/jump, and when combined with the left analog boost, can be strung together into combos to keep the player off the ground entirely, which is a necessity for some levels. Face buttons are used to select weapons, and the right trigger fires the selected weapon.
The game comprises ten levels, the story unfolding via text between levels, or the occasional in-game cutscene. At the beginning of most levels, excluding boss battles, you have the choice of picking either Kelly or Saburouta. Kelly's primary weapon is fast but weaker, and she's overall faster and more maneuverable. Saburouta is slower, has more limited maneuverability in the air, and carries a stronger primary weapon, the Matchlock Cannon. Basically the two characters make up the two difficulty settings, with Kelly being the "Normal", and Saburouta being the "Advanced" setting. The real difference being that Kelly's style allowed several lock-on targets, while Saburouta's weapon splashed and was aimed at specific targets. Because of this, Kelly's gameplay is more frantic and requires less direct focus, while Saburota required specific aiming and direct attacks to be effective.
The artistic style of the game is similar to the steampunk sub-genre of fiction, but the game's designers prefer to call it "elec-punk," described as a step even further, with the harnessing of electricity for imaginative new uses. The game's stages are divided into indoor mechanical looking levels, with lots of gears and ornate metalwork, and outdoor stages, set in very organic looking valley's, craters, and other fantastic otherwordly locations.
The divergence between the storyline and the real world is sometime in the 19th century. A scientist named Dr. Hebbel, referred to as "The Great Genius", discovers a wide range of scientific breakthroughs, including fusion and nuclear power, genetic engineering, computers, space travel and countless others. With these technologies the British Empire quickly conquers the Earth, and he is regarded as a god by everyone on Earth, even to the point where when he speaks out against Queen Victoria she is overthrown and he is chosen to lead. He institutes a variety of radical reforms, bringing humanity up to modern societal standards (e.g. multiculturalism, sexual freedom, equality)[. He disappears shortly before the game begins. ]
The game itself follows members of an elite military force known as "Team Dolphin" (established 1887) as it tries to defend a colony world. The two playable characters are Kelly O'Lenmey, who was born in Ballymun, Ireland, daughter of Dr. Hebbel and Saburouta Mishima, grandson of one of the Samurai rebels who aided the British in conquering Japan. The story takes place on the planet of Tir na Nog, with the player facing off against thousands of alien creatures that resemble giant insects.
GunValkyrie was originally developed for the Sega Dreamcast. Its visual style was very similar to the final Xbox version; though the characters were rendered using a slightly cel-shaded style reminiscent to Jet Set Radio, a popular Dreamcast game also made by the development studio, SmileBit. The Dreamcast version's distinguishing feature was that during the game's development, GunValkyrie utilized a unique control scheme using both a light gun and a controller. This control-scheme was dropped when development shifted to the Xbox version.