Rewatching this battle, I've come to the conclusion that Ash beat Gengar easily because of Confusion, sure I do wish the battle was longer, but I'm going with Confusion doing a critical hit on Gengar that really lowered its health with likely another critical hit tackle.
To be honest this was probably the most faithful adaptation of a game battle that I can recall or have seen. Everyone's health was pretty realistic for the games, only taking a few hits depending on the attack. Noctowl learning confusion was probably the best adaptation of the games where you can learn a new move upon leveling up.
Sure there were some anime only concepts, sure, like Haunter's hands being used to catch and toss Cyndaquil or even the ghosts invisible. But it was pretty straight forward of an adaptation for someone who either has low leveled Pokemon or high leveled Pokemon going against a Gym leader.
Noctowl using Confusion all of the sudden was hax. I always hate when new moves are learned without a reason, but Morty handled Ash's hax pretty well for the most part.
I don't know if it was changed and wasn't applicable in 2nd gen, but unlike evolution you technically can learn new moves in battle, sure you have to beat an opponent first for that to work, but one could argue in this battle specifically battling Gastly gave Noctowl enough experience, though yes I do realize it should've had it in the beginning (I guess) though in comparison its as simple as just not choosing confusion until the last moment which would work LIKE this in terms of anime.
Of all things, this is the least Deus Ex Machina thing in a Pokemon anime battle since its PERFECTLY plausible to happen within the games. Just imagine Noctowl got a lot of experience attacking Gastly in the previous episode, and Cyndaquil beating Gastly gave it 100 experience points to reach a level at which it learns confusion. Which was only applicable to the end of the fight when Noctowl learned "confusion" as in "hey trainer I know you don't have text alerting that I learned confusion like in the video games, which technically doesn't make sense without some weird headcannon being real but hey I know confusion."