I think the old interpretation of GB was "By Decree of the Council".
In fact, these were again numerals, 3 and 2, and the meaning was "Capital of THREE provinces, holder of TWO neocorates". When Tarsus acquired a third
neocorate under Valerian, the abbreviation changed to GG.
We owe this discovery to P.
Weiss, Die Abkürzungen GB und GG auf den spätkaiserzeitlichen Münzen von Tarsos und Anazarbos,
Chiron 9, 1979. I know Weiss' conclusion from R.
Ziegler, Städtisches Prestige und kaiserzeitliche Politik, Düsseldorf 1985, p. 27.