The Gamma Trianguli sector was a region of space in the Alpha Quadrant. Named after the star Gamma Trianguli, this sector contained Archer's Planet and Gamma Trianguli VI. (TOS: "The Apple"; ENT: "In a Mirror, Darkly, Part II", display graphic)
The Star Trek: Enterprise fourth season episode "In a Mirror, Darkly, Part II" revealed this sector reference, as well as the planet's name, in text appearing in Jonathan Archer's personnel file. A more detailed look at the original production artwork can be viewed on the Twitter timeline of the episode's writer, Mike Sussman.
"The Explored Galaxy" was a map of charted space in the Alpha Quadrant. Gamma Trianguli was located between the Tholian Assembly and Romulus. Both locations were identified in DS9: "The Search, Part II", "Call to Arms" as being located in this quadrant. This star chart, first seen chronologically in Star Trek VI: The Undiscovered Country, was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were: TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game" and DS9: "In the Hands of the Prophets", "Cardassians".