Composer/Creator Brooke DeRosa has an obvious affection for, and understanding of, the mix of catchy melody and clever lyrics that made Gilbert & Sullivan’s works audience favorites for well over a century. But rather than contenting herself with placing PIRATES OF PENZANCE on the next SpaceX shuttle, DeRosa has created her own homage to G & S, with GUNFIGHT AT THENOT-SO-OK SALOON. And a very fun evening it is.