Scarf Gengar is annoying... it catches my Starmie by surprise :(
Gengar outruns Modest Chloro Venusaur and +2 Jolly / Naive Cloyster, which are important to note.
Personally, I think HP Ice > Thunderbolt, since otherwise Gengar cannot OHKO Dragonite and Salamence, which are more prevalent threats to check than Gyarados.
HP Ice / Thunderbolt
Trick / Thunderbolt