Actually, that's not a problem at all. Clearly, Christians think there are things God cannot do (anything against his benevolent nature). I could perhaps argue that doesn't make him the GCB, but otherwise there doesn't seem to be a problem there specifically.
However, the problem is I don't think it makes sense to say you can conceive of a being who can conceive of that which is inconceivable by definition. Does it? Otherwise, my GCB can conceive of a square-circle.
However, the problem is I don't think it makes sense to say you can conceive of a being who can conceive of that which is inconceivable by definition. Does it? Otherwise, my GCB can conceive of a square-circle.