Sector G-5 was a region of space, a sector located somewhere in the galaxy's Alpha or Beta Quadrants, a location on star charts used by the Federation in the 23rd century. The advanced Carnak civilization resided in this sector.
In 2267, while exploring sector G-5, the Federation starship USS Enterprise was bombarded by meteoroids launched by a cloaked Carnak space cannon mass driver. Heavy damage resulted in the vessel being caught in the planet Carnak's gravity well, and the starship was forced to employ vertical thrusters and land on the surface. (TOS comic: "Where Giants Tread")