Taking place concurrently with R-Type Delta, Gallop sees the player as a member of the like-named private security organization tasked with hunting down autonomous vehicles driven berserk by the Bydo, dubbed "Mad Cars."
The player pilots an R-Type fighter, the R-11B Peace Maker. As in most horizontal shooters the screen scrolls continuously to the right, but Gallop is unusual in that the scroll rate can be controlled by the player. The further right the player's craft is oriented onscreen, the faster the game scrolls.
Gameplay progress through five different "zones," each culminating with a boss. Significant bonus points are awarded for completing the zones quickly.
The R-11B is armed with forward-firing guns, as well as a lock-on laser that automatically targets enemies, but whose energy depletes as it is fired. These weapons can be powered-up twice by picking up a letter "P" and the laser's energy can be quickly refilled by picking up a letter "L."
Picking up a letter "M" will equip the R-11B with a missile unit that will fire one of three types of munitions: anti-air missiles, anti-ground missiles, or napalm bombs. Contact with any terrain will cause the missile unit to be lost, however.
Source: Wikipedia, "Armed Police Unit Gallop", available under the CC-BY-SA License.