Gateway is an ancient class L planet located in the Beta Quadrant, and the home of the Guardian of Forever. The planet is also known colloquially as The Rock of Ages, Forever World or simply the Time Vortex Planet, among other names.

The Guardian of Forever is a construct of an unknown, ancient alien race, that functions as a time portal, a gateway to the time vortex that allows access to other times and dimensions. It is located on an ancient planet where the focus of all timelines throughout at least the Milky Way Galaxy converge. It is apparently sentient, responding to external stimulus such as questions and actions, and can even somehow control the flow of time. It generates immense ripples in time that manifest themselves as spatial disturbances in the region around the planet where it is located.

The Guardian is located among the ruins of a large, forgotten city that stretched beyond the horizon in all directions around it. Based on initial observations, the ruins appeared to be at least one million years old.

It should be noted that the Guardian claims to be on the order of at least five billion years old.

Capable of speaking to those around it, the Guardian explained that it is "its own beginning and its own ending," and that, "since before your sun (Sol) burned hot in space, [it had] awaited a question." Apparently an inert formation of quasi-metallic substance, the Guardian creates portals to other times.

Discovery
The planet was first discovered by the Federation in 2267 when the USS Enterprise traced waves of temporal distortion back to the Guardian. A vast area surrounding the Guardian was filled with the ruins of an immense, forgotten city that spread past the horizon in every direction. The ruins appeared to be at least one million years old, though the Guardian itself was much older.

By 2269, the Federation had sent a research team of historians, scientists, and engineers to explore the planet and study the Guardian.

Further information
While the Guardian of Forever was constructed on the planet circa 5 billion years ago by an unknown race, it was not until a few hundred thousand years ago that Gateway gave rise to its own intelligent species, who would become known as the Originators. Their large cities spread across the planet; one of the largest was Oyya, which spanned over 2,000 square kilometers around the Guardian.

Eventually, the Originators disappeared and their cities turned to ruins, yet the Guardian still continued to exist at the center of the ruins, and awaited to be asked a question again.

After the Enterprises discovery of the Guardian, Gateway was declared a Federation Protectorate and all information about the planet and the Guardian was classified at the highest level. A starship was assigned to constantly patrol the sector, though the crew did not know what they were defending. The Ellison Research Outpost was also established to study the Guardian; however, the research teams had only sporadic success in getting the Guardian to answer their questions.

In 2270, Kirk, Spock, and McCoy returned to Gateway in order to rescue Zar, Spock's son from Sarpeidon's distant past. Fifteen years later, a Romulan invasion force attacked Gateway. While the Enterprise and the USS Lexington kept the bulk of the forces at bay, Tal and his advanced guard were stopped from destroying the Guardian by Kirk, Spock, and Zar.