system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel] How did Dr. Doom become God Doom, and how does God Doom differ from Dr. Doom?

Also, which universe is God Doom a part of?
gpt
Spoilers for the current Secret Wars crossover follow - 

We don't know exactly how, yet. The Beyonders - an extradimensional race of omnipotent beings, of which the one from the original 1984 Secret Wars was a newborn infant - decided to destroy the entire multiverse, for reasons we don't know. Dr. Doom engaged in a fairly convoluted plan to stop them... by destroying the multiverse himself, before they did. 

The difference being, the Beyonders were going to completely wipe everything out. Doom saved bits and pieces of worlds from across the multiverse. Then, somehow - we still haven't been shown how in the comic - he managed to defeat the Beyonders and steal their power, then assemble all those bits and pieces of parallel Earths into a single world, Battleworld. 

Then he and Dr. Strange altered the survivors' memories to remember that the world had somehow always been this way, made up of little patches of alternate universes, and that Doom is God and always has been. People mostly just don't think about it too much, and questioning it is considered heresy.

So the answer to which universe God Doom is from - lots of them, in one sense, in that he is God over a world made from the Earths of many different universes. None of them, in the sense that no actual universe survived, and Battleworld hangs in an empty void, circled by a sun made from the Human Torch. But in terms of origin, he is the Doom of Earth-616, the main Marvel Universe.