Gog was intoduced in "The Kingdom," which was a lameass, worthless, POS excuse for a sequel to Kingdom Come. It was basically a story that tried to fit Kingdom Come into the DCU continuity by showing it to be a "possible" future contigent on hypertime. You see, hypertime was basically DC's way of reintroducing the multiverse. An endless stream of possibilities where anything could happen (this is where Elseworlds apparently came from, even though Elseworlds stories were written years upon years before DCU came up with this hypertime business).
Anyway, Gog was this kid who was saved by the Kingdom Come Superman and as a result, grew up worshipping the Man of Steel. He was so zealous that he built a Superman church. Superman showed up, gave him the whole "I'm not God, stop worshipping me and do something worth a crap with your life" speech and then Shazam and a bunch of other immortals decided to endow this human with superhuman power (I don't remember they're rationalefor this, but I do remember it being stupid). So Gog decided to spend the his life going back in time one day at a time and killing Superman over and over and over again. Later he steals the child Superman and Lois have together and names him Magog. It appears that this Magog child will then become the Magog we saw in Kingdom Come, but its never fully explained, and by the end of the story, you probably won't care anymore because there's a huge chunk of it where the big 3 (Supes, Bats and Wondy) don't even appear, and instead we're treated to side stories about the children of superheroes; most of which suck. The story about Ibn X'Uffasch, the son of Bruce Wayne and Talia al Ghul, is the only worthwhile part of the whole stupid arc.
Anyway, since The Kingdom links KC Continuity and the "real" DC continuity together, Gog has been (unfortunately) permitted to appear in Superman comics since, but as far as I know, Magog never has.