Almost every Gerstmann's q I have seen so far involves the patient not being able to perform calculations. The lesion affects the Angular Gyrus, so the acalculia is kind of a dead give away. Other than that, just try to memorize (roughly) the regions that each artery supplies. Just know ACA, MCA, and PCA and you'll be fine. But, I don't think there is any easier way. Good luck!