The technology that generates a Gellar Field around a starship intended for faster-than-light travel was invented along with the Warp-Drive sometime in the 15th Millennium AD during the Dark Age of Technology. The technology, called a Gellar Field Device, allowed Warp-capable starships and their occupants to survive the extremely hostile environment of Warpspace, also known as the Immaterium. The Gellar Field protects the starship and its occupants from the hostility of the psychically-reactive Warp itself as well as from the predation of Warp entities such as daemons.
Gellar Field Device
The Gellar Field Device emits an energetic, invisible force field comprised of unknown subatomic particles called a Gellar Field, which essentially maintains a bubble of real space-time around a starship travelling through the Warp using the power of its sublight drives after using its Warp-Drive to pierce the veil between the physical universe and the Warp. Since entities native to the Immaterium cannot breach its boundaries to enter the physical universe except under certain very specialized circumstances, creatures native to the Warp are repelled from the boundaries of the Gellar Field. The weakening, failure, or collapse of its Gellar Field while a starship is travelling through the Warp would be completely disastrous, as was discovered by the crew of the Imperial frigate Eisenstein during its famous flight to Terra from the Battle of Istvaan III to tell the Emperor of Mankind of the betrayal of his favoured son Horus. Warp entities would tear the starship apart to reach and consume the souls of the crew. As such, the field must be active at all times. For the field to work, the starship must be entirely sealed; no open entrances that would breach the field's protective envelope are allowed.
Orks do not seem to generate a Gellar Field around their admittedly patchwork spacecraft and their method of surviving transit through the Warp unharmed is unknown, though it is possible that their latent, collective psychic powers help make a repulsive Warp barrier instinctively. It is probably safe to assume that other intelligent alien races have their own method of shielding their starships if they travel through the Immaterium, be it a variant of the Gellar Field or some other method which replicates the same protective effect. The Necrons do not travel through the Warp at all when they use their unknown phase technology to move around the Milky Way Galaxy, so they have no need for a Gellar Field alternative. This is also true of the Eldar and Dark Eldar when they use the Webway, however they may use a much more advanced variant of the technology to create and maintain the Webway of Warp tunnels that they use to travel interstellar distances which are themselves shielded from intrusion by malevolent Warp entities. The Tyranid Hive Fleets do not travel through the Warp, either. Instead, they use Bio-Ships called "Narvhals" that manipulate their intended destination's own gravity to compress Realspace into a type of gravitic transit corridor or worm hole, allowing the Hive Fleets to cover vast distances very quickly. This method grows far less efficient as they draw closer to their destination, extending the travel time by years, or even decades, as they must slow to more conventional methods of propulsion as they near a star system. The Tyranid Hive Mind's psychic Shadow in the Warp makes Warp travel and astropathic communication much harder if not impossible for Imperial starships and psykers. It is possible the massive psychic presence of the Hive Mind in the Warp disrupts both the Immaterium's local geometry and possibly an Imperial starship's Gellar Field.
Note, the Gellar Field is by no means infallible; indeed, it has been broken on many occasions, to the great misfortune of a starship's crew and passengers. The commonplace disappearance of starships in the Warp is often attributed to Gellar Field failure and the Imperium has come to accept that a small but significant percentage of its interstellar traffic will be lost to accidents of this kind.