After Whitney, I got rid of Noctowl and proceeded straight to Ecruteak City
After defeating all the Kimono Girls in Ecruteak, I challenged the Gym

Pretty easy Gym, with Morty's Haunters helping to defeat themselves with Curse
Snubbull would use Bite, bringing the Haunter to half of its health, then the Haunter would use Curse, knocking it out
Gengar missed all its Hypnosis attacks, which gave me a good chance to bring it down
Snubbull evolved after the battle