If ur Gengar has Shadow Ball and not Sludge Bomb I'd include it in your lineup, swapping the Gyarados since it has the lowest attack and doesn't get STAB from its super effective move unlike your other 5 Pokemon, so I think your Gyarados is the weakest Pokemon in your lineup.
And if the enemy Gengar has Focus Blast your own Gengar is triple resistant to it which somewhat makes up for the lower stamina compared to Gyarados.