The Geminus Gulf was a region in the Beta Quadrant comprising three sectors of space near the common border between the United Federation of Planets and the Romulan Star Empire. Sparsely populated with stars, at the astrographic center of the Gulf was the Chiaros system, containing the region's only known inhabited planet, Chiaros IV.
Because of treaty provisions, neither the Federation nor the Romulan Empire could expand into the Gulf, unless first invited by a local species. Hence, Chiaros became of great importance to both powers in the early 2370s.
In violation of the treaty, the Romulans constructed a cloaked starbase near Chiaros, and destroyed the USS Slayton while trying to subvert the Chiarosian government. The Romulan agenda for the Gulf was disrupted by the USS Enterprise in 2373. (TNG novel: Rogue)
Timothy Sinclair later theorized that the Tkon Empire may have used the Starkindler device to relocate stars from the Geminus Gulf, accounting for the unusual sparsity of stellar bodies in the region. (Star Trek: Pendragon: "Starkindler")