Yes, I voted for Gastly, as many as you did. But I didn't consider anything about Mega Evolution and a pure Grass-type Pokémon or such. Easy. Gastly resists Oddish's Grass-type moves, and also greatly resists Oddish's Poison-type moves (its two types). While, Oddish doesn't have any weakness to Gastly's moves, it doesn't resist its types' moves either.
Also, remember the battle was Gastly vs. Oddish, not Mega Gengar vs. Bellossom.