Gergis was the birth place of the Herophile sybil, who is depicted on the obverse of this coin. She may have been one of the Erythraian sibyls, who presided over the Appollonian Oracle in Erythrai. However, Pausanias mentions a Herophile sybil in Delphi, who was the daughter of Poseidon and Lamia, who was a son of Zeus. Legend has it she was the first woman to chant oracles.
He goes on to say that Erythrai laid claim to her, though evidently Gergis did too (unless the image is of someone other than Herophile).