The Gravitic Drive is a method of interstellar travel, most famously used by the Tau.
While working under the same basic principals as the Imperial Warp Drive, there are several key differences. Primary among these is that the ship that uses it does not actually enter the Warp proper - rather, it enters the void that exists between realspace and Warp space and are then flung back into normal space at faster-than-light speed, akin to forcing a ball underwater and releasing it. Compared to the Warp drive used by most other races, Gravitic Drive technology is significantly slower. However, as the ship does not actually enter the Warp, it is not prey to the dangers within.[Needs Citation]
The Tau are the most significant users of the Gravitic Drive. They already had a primitive form of this technology in M39, using it on sub-light craft to colonise the other worlds in the T'au system. This system projected a sheath of gravitic energy ahead of and around the vessel which was continually re-projected further ahead, drawing the ship behind it rather like an archaic sail.
By seemingly random fortune, a survey on one of T'au's moons uncovered the remains of an alien vessel that sported a Warp drive. The Tau, lacking psykers as a race, instead reverse-engineered this design and incorporated it into their already existing technology.