Battlestar Gemini is actually one of the smallest Battlestars that are Colony Flagships but has the most armour relative to her size. She was comissioned 12 years before the Fall, and is presumed destroyed along with the other Battlestars in the counterattack.
Her armament is unkown but from the Colonial Database we have this information:
(Pic copyright David "CanisD" Breidis)
Battlestar Gemini has the regular Valkyrie-Class set up, while adding additional armour and another missile pod on each side. It also boasts a larger-than-usual Viper contingent.