The Guardian's planet was never named. We don't even know in which quadrant it was.
Whut?
They're using the Guardian of Forever? I hope Harlan Ellison's estate charged them $$$$$$.
As for which quadrant... this is another way in which TOS doesn't mesh with the other series. Roddenberry's writers tended to use the names of well-known bright stars, because they assumed that at least some of the viewing audience would have heard of them. They actually did use a research company to vet the scripts, but of course were not obliged to follow their recommendations if it turned out that the scriptwriter had made a major scientific error.
So now that we have modern knowledge of a lot of these stars, it renders some of the TOS episodes scientifically laughable from an astronomy pov. There is no way that there could be indigenous intelligent life on any planet around Vega, for instance, since Vega doesn't even have stable planets yet (RL; since Vega is only 26 light-years away, for all intents and purposes, studying that star is almost like doing real-time observations - we're seeing it as it was 26 years ago, which is a tiny drop in the bucket of stellar lifetimes, even those as short as supergiant stars).
According to Memory Alpha, the Guardian of Forever was located either in the Alpha or Beta Quadrant.
Memory Alpha is a terrific resource site, btw. It's basically the Wikipedia of Star Trek.
EDIT: I meant to say also that using real stars meant a mishmash of using ones that were close to Earth (the aforementioned Vega) or far away - certainly too far for the Enterprise to just sashay over there in a few hours or even days. If memory serves, there was an essay in one of the Best of Trek collections about that - pointing out that the Enterprise couldn't possibly have visited all those solar systems in the same universe (yet another reason for the author's contention that some of the TOS episodes took place in alternate universes).
TNG solved this problem by using made-up names for planetary systems. Of course they didn't bother checking previous scripts to see what had already been used, so we got the ridiculous situation of the Telurians, the Telarians, and the Telorians, who were definitely not the same people, and the episodes had nothing to do with each other.