This is a very weird core since I almost never see Gengar and Gastrodon on the same team together. Generally I see Gengar on sand offense teams since it is immune to Fighting and Ground-type moves which threaten Tyranitar, and is much faster than many Pokemon commonly found on sand teams (both inducers, Ferrothorn, Forretress, Gliscor, Heatran, etc.). Gastrodon is generally utilized to put a stop to rain teams, but outside of combatting Politoed armies it often fails to pull its weight, because it often struggles against Sun teams and it can almost never sweep on its own, because it is better suited as a defensive Pokemon.
To find out how I'm thinking of pairing these two mismatches together, watch this thread over the next few days.