WHAT first century GB? The older men in Jerusalem? I can't help but think that they were just the remnants of the Jewish faction, though christian, and spoke mostly for the jewish converts; they campaigned for circumcision and for observance of portions of the law.
Is the term GB used in the body at all? Even in the WT quote, the term GB is not used; instead it is:
the apostles and Jerusalem elders
When read in that way, it doesn't sound so much like the modern day to me; just one faction of the first century body of believers who wanted to influence what the christians needed to observe. Their pro-Law bias is evident in wanting to preserve the ban of eating of animal blood and on circumcision. Later they influenced Paul to go to the temple for ceremonies involving young men in order to convince the Jerusalem christians that Paul was not in favor of abandoning the Law ( he of course was) and Paul went along with it, even though in Paul's letters he says that the law was taken out of the way by Jesus' death.