In last sundays WT they explained, atht the appointment of elders by holy spirit happens, because a brother must fulfil the requirements outlined in the inspired word of God.
So were are the requirements for GB members in the scriptures ? If they are not there (obviously, since even a GB is not mentioned), this means, that the appointment must happen directly, like God or Jesus speaks to them or writes them an email (yes jehovahs chariot is moving in line with newest technology).
I beleive, with an answer to this question the whole thing stands or falls....