Jupiter (Saturn in the novel).
He then evolved into the Star Child.
Relatively little is told of the Star Gate in the film, but it is likely to say that, like the monoliths, it existed in the Solar System about three or four million years ago, as it still does today. Of course, the technology which creates the Star Gate has likely existed long before, with the alien race that created it.
The novels in the Space Odyssey series have more to say on the Star Gate. In 2001: A Space Odyssey, the only novel in which a human, namely Dave Bowman, actually goes into it, we are told by the narrator that TMA-2 (inside of which was the Star Gate) was indeed placed around the same time as the placing of TMA-1, on the moon Iapetus, which is one of Saturn's moons. The narrator then goes on to say that another moon of Saturn's was destroyed in order to produce the Star Gate. While the actual purpose of it is not made clear, it is suggested that it is an interstellar highway, which various ships use as a means of travel, as well as an automatic trap set to capture David Bowman.