Gamma Hydrae (often spelled Gamma Hydra) is a spectral class G8 star 133 light-years from the Sol system.
In 2381, during the short conflict between the Romulan Empire and the Federation, Romulan forces broke through the lines and attacked the fourth planet in this system, They then landed troops to occupy the colony there. Starfleet soon retaliated by sending two Marine Expeditionary Brigades, and the entire Second Fleet to retake the planet. (Star Trek: New Order: ...The Hardest Part)
A battle was fought between the USS Excalibur and Klingon ships at Gamma Hydra in the 2260s. The Federation ships that came to Excalibur's aid were the USS Athena and the USS Joshua Chamberlain. (Avalon Universe: Air and Darkness)