Dating back to the Martin Lawrence television series "Martin" (1992-1997), this phrase refers to Martin's partner Gina, who is described as his "more level-headed, longsuffering better half." Martin used this expression to respond, exasperatedly, to Gina's criticisms or expectations, which were often quite reasonable on her part.
The phrase can also be used to express, in a positive light, shock and awe - much in the manner of "damn girl, you looking fine!" It is similar to other "whoa!" type exclamations that are meant to convey that one is taken aback.
Damn Gina! (works best if the recipient is aware of the expression's context in the history of the 1990s Martin sitcom)