There's a few other reasons not to expect there to be something like GOD for magic.
For starters, GOD can work as a distributed global network for the matrix, because everything they do takes place on the Matrix, which is a global network. A GOD agent in Tokyo can police a Matrix site in Boston without any difficulty. He can go wherever he needs to, if the matrix node that he is trying to get into remains connected to the matrix. If it becomes disconnected, then the problem he was responding to has, by default, ceased to be a problem. If he needs to work with other agents, they can come together from every corner of the earth near simultaneously, and needing to physically go to the site of anything they are dealing with is an extremely rare occurrence. Most of the time, if there is physical, meat-space evidence they require, they can just tell the local authorities to gather it and ship it to them. If they need a physical presence to deal with a matrix threat, they take over a nearby drone, or order locals to do what they want. But why would they need that? After all, they can just make sure that the perpetrator is fried by black IC and attack programs, and go back to wandering their corner of the digital world.
Magic doesn't work this way. Even if you can move quickly in the Astral space, you still need to know where you are going (which does take at least enough time to be informed in the meat world, and then project, and then arrive), and showing up in the astral doesn't help you do much if the things you need to be responding to are in the physical world. Even if things are in the astral, closing down an area of the Astral for a short (or long) period of time is not all that difficult (Offensive Mana Barriers are easy to throw up and over-cast to near insta-gib levels of power for Astral form Magicians). Physically travelling to a location is usually as slow for a mage as it would be for every other person, so if you need to do that, good luck responding in a timely manner to anything but the most local of problems.
The second reason to avoid it is one of how long lasting the problems in each realm can be. A particularly nasty and dangerous AI or virus in the matrix can and will last forever, if no one actively deals with it. Responding early and hard becomes important, because they can even spread. GOD does this well. It is not widely believed that similiar effects can happen often with magic. Sure, a magician might summon a spirit or cast a nasty spell, but they do it and it is done, as far as most shadowrun historical occurrences show. The results might linger and be problematic (the spirit hunting and possessing people or summoning more of it's kind, or an area of the world being destroyed or becoming uninhabitable), but the effect that created it is typically over and done with, and now you've just got to deal with the aftermath. Most known magical effects are not persistent or capable of propagating themselves... or, if they are, they have to be dealt with as individual occurances, often in meat-space, which creates the logistics problems I mentioned above.
And the third reason one hasn't arisen is the question of what the corporations would have it protecting against. GOD has very specific areas of responsibility that they operate in. They don't care that you're making snuff BTLs or operating a bunraku parlor... they only care if you are doing something that could disrupt or endanger the users of the matrix, which all the corps universally agree is Bad For Business. While we players can look at the mechanics and the dark secrets of the world, and know that Blood Magic is detrimental to the survival of all of Metahumanity because of what it is likely to unleash, there is no consensus of that in world, because the vast majority of people don't know it. Blood magic is viewed as illegal most places because it violates other standing laws and morals: You're committing murder and/or assault, often combined with kidnapping and other mundane crimes.... but it's fine in Aztlan because the laws there have been changed to accommodate it, and the Great Ghost Dances, while technically blood magic and just as dangerous to meta-humanity as anything Aztlan is doing/has done, are regarded in an entirely different light, thought of as a sort of noble tradition of self-sacrifice. So the NAN and Aztlan aren't going to agree to establishing a group that is charged with "Preventing all blood magic," even if we as players know that they should.
You really have to deal with all three of these reasons before you would see a GOD equivalent arise, probably in reverse order: Is there a charge that could acquire sufficient backing in universe? If so, is there actually anything the group could do about it? And how would it be logistically feasible for them to do those actions, if there is anything they can do? I admit I'll be curious to see what the proponents of such a group have for answers to those questions.
