Looking at the tier list to try and find a 6th member. Saw Gastly (Trade) at A.
Am I missing something here? Haunter is (rightfully) all the way down to E. Gengar getting the elemental punches doesn't change the fact that until Lv. 21 (Which is after Sudowoodo, mind you, right at the mid-game) Gastly's only attack option is...
Then it gets Night Shade.
I can't in good conscience advocate for a literal dead weight for half the game to be sitting at A.
I haven't finished the run but I'm well past the point of early game Gastly, and it hasn't ever felt bad once. I caught it in Violet City and while it has needed some support, it's been consistently solid as a team member and as soon as it hits Gengar it's great.
Lick is definitely a bit of a bummer, especially since it means you can't hit the Normal types you wall (and there's a fair few). But the Paralysis chance is nice, and Lick's low damage is less of a problem when hitting the several early game Mons with only normal attacks. It also can't be stressed enough how solid Level 1 Hypnosis is, cheesing bosses and making catching way easier.
Speaking of cheese, Curse is a great way to speed up the matchups that can't hit you effectively. Not just immunity, either: Bugsy's rather scary Scyther deals 1/4 damage with Fury Cutter, which makes Hypnosis+Curse easy to pull off. Stupid AI that refuses to switch makes Gastly actually surprisingly effective.
Once Gastly gets Night Shade, it continues to be the same type-resisting, speedy sleeping, Cursing cheeseball but with consistent damage, usually a 3hko, useful for catching Pokemon with predictable outcomes. Miltank still threatens with strong Rollouts but a Hypnosis or Curse at least puts a timer on the match.
You get Gengar right around when you reach Morty, immediately able to hit it with a Punch TM or three depending on your team needs, while STILL being a fast sleeper, STILL resisting everything, and now hitting with excellent coverage and power. I've had far less effective early game Mons with way worse late-game payoffs.
Also, while this is pretty niche, it's also around level 40 not too long after you face Suicune, meaning it's an ideal Entei/Suicune hunter with high speed, Mean Look and Hypnosis. There's a mathematical way to hunt down Entei and Raikou in about 15 minutes with Repels, and they're debatably worth the detour, just to get them out of the way if you want Ho-Oh or want some Electric and Fire Pokemon. Not efficient or optimal, but fun, and a natural option when using Gengar (or Jynx). EDIT: After completing catching these two (about an hour combined, maybe) I looked at Entei and Raikou's level-up movesets and... probably not gonna use them in the run lol Still fun tho
Give it a try! The Gastly phase is absolutely manageable and the flexibility of Gengar is worth it. Not optimal, but well above the overall efficacy of anyone in B imo