I don't think you fully understand what the argument is saying, Roach/Matt.
If you say the GCB doesn't exist in the real world, the argument says you aren't thinking of the GCB. The GCB, it says, must exist because it is greater to exist than to not exist. Furthermore, if you are saying GCB/God defined that way doesn't exist, you are comitting a contradiction. That is saying that God isn't really God or that the greatest conceivable being is not really the greatest conceivable being. Therefore, God must necessarily exist.
What you need to think of is whether or not the GCB can exist, is it possible?
Furthermore, is the argument circular in that it assumes God can possibly exist (to be defined) to prove his existence, or that it uses a concept to prove a concept or being can exist. Acquinas' main objection seemed to be this. Also, he argued that God wouldn't really have to exist in the understanding since we cannot fully understand God and to Atheists (the people of whom this argument is directed at) do not conceive of God and therefore deny him. He applied the logic of the argument to itself, a sort of anti-ontological argument. If something cannot be conceived of, it cannot be said to be self-evident. He said God existed, but for different reasons (empirical ones).
Can God conceive of a greater of himself, even if he doesn't exist? If not it would appear he isn't really God and that a GCB by definition cannot exist.
Is it a contradiction to say that God doesn't exist if you prove he cannot exist?
Why couldn't Satan be the greatest conceivable being?
Is the GCB even really God.. that is is the conclusion a moot point?
Can the definition of something be used to prove its existence or does definition presuppose existence in order to define(see: circular objection)?
Is existence a trait? Does having existence make one superior to that which does not exist. Does it add to the concept of the being? If not it would appear one cannot contend that to be in existence is really greater than not existing.
Can you make the conclusion and absurdity clause themselves absurd by applying the logic of the argument to it? That is can you say that God wouldn't really have to exist in the mind just because we hear the word God. Or: Does denying God even mean conceiving of God? Does conceiving of something mean it exists in the understanding?
The argument has some problems and it ultimately boils down to a word game of definitions. The argument in every sense of the word tries to define God into existence by his definition.
I am an atheist and it took me awhile to come up with a way to completely destroy these arguments, and I had to read the works of Thomas Acquinas to do that.