From Halopedia, the Halo Wiki
The UNSC Gettysburg was a Frigate in service with the UNSC Navy during the Human-Covenant War.
On July 17, 2552, the Gettysburg, along with the UNSC Alliance, responded to a distress signal sent by Commander Jacob Keyes from the Sigma Octanus System. The vessel later fought in the Battle of Sigma Octanus IV.
The vessel then returned to the orbital space dock of Reach, where it was heavily damaged in the Battle of Reach. There the entire crew was killed in the battle.
Eventually, SPARTAN-117, along with several survivors of Battle of Installation 04 and Battle of Reach, used the Gettysburg, fusing it with the Covenant ship Ascendant Justice, thus becoming the first Human-Covenant hybrid vessel in history . It was used because the Ascendant Justice could not do the two Slipspace jumps in and out of a group of Covenant ships fast enough using just its own reactor. 
Eventually, the vessel was unlinked from the Ascendant Justice so that Vice Admiral Danforth Whitcomb and First Lieutenant Elias Haverson could use the Ascendant Justice to lure a Covenant battle fleet to the Unyielding Hierophant, which was about to explode. 
Return to Sol
After Operation: First Strike, the Gettysburg and its now extremely small crew headed back to the Sol System, Earth's home system.
After that, its fate remains unknown. Certainly the slipspace drive captured from Ascendant Justice was likely studied, though whether improvements were made in time for the Battle of Earth is unknown. The actual hull may have been refit for service in the decimated UNSC Fleet, or scrapped to provide materials for other ships.
The Gettysburg was refitted with the slipspace engines from the Ascendant Justice prior to unlinking.  Since Covenant Slipspace engines had always been faster than UNSC engines, the Gettysburg then became fastest ship in any of the Fleets of the UNSC Navy.
- It is named after the Battle of Gettysburg. The American Civil War battle took place on July 1-3 1863, in Gettysburg, Pennsylvania, on Earth. This is interesting, in that it was fused with a Covenant vessel, just as the two factions of the American Civil War eventually rejoined into one country.
- It is possible that the Gettysburg may have joined the strike on the Forerunner Dreadnought with the Aegis Fate and Forward Unto Dawn during the Battle of Voi after repairs from heavy damage.
- In a Star Trek Armada II Shippack including UNSC Frigates, the Gettysburg was released as a hero ship, along with the UNSC Forward Unto Dawn. Its special improvement is quicker impulse and warp engine speed.
- ↑ Halo: First Strike, page 208
- ↑ Halo: First Strike, page 209
- ↑ Halo: First Strike, page 332
- ↑ Halo: First Strike, page 329