If GCB is logically possible then it logically cannot fail to exist. Therefore, it is logically necessary that GCB exists. There is a possible world in which maximal greatness is instantiated. Necessarily, a being is maximally great only if it has maximal excellence in every possible world. Necessarily, a being has maximal excellence in every possible world only if it has omnipotence, omniscience and moral perfection in every possible world.
Maximal excellence is instantiated in every possible world.
Therefore, in the actual world there is a being that is omnipotent, omniscient and morally perfect. However, Plantinga is a traditional theist, not a deist. His argument is included simply because it is another example of a valid ontological argument. GCB's perfection does, however, require a supreme degree and scope of power. The point of contention between Plantinga and deism is his belief that supreme power should be construed as omnipotence.