This thread is pretty much dead, but I guess potential tier changes are interesting even if the OP isn't here to really do it. As for the subject of the tier change, if you're catching and using a Gastly as early as possible, then I think you're doing it wrong. Until either it reaches level 29 or you reach the Route 212 tutor, its best attack is night shade, so you'd want to get it right before one of those two events to minimize the exp. share training. This is the best way I know to use a Gastly (with trade, of course):
When you reach the Lost Tower, you can go inside and catch a level 21 Gastly.
When it reaches level 29, it learns shadow ball and can be evolved into Gengar. You can also teach it hypnosis, focus blast, or thunder around this time.
When you reach Celestic Town, pick up the choice specs.
When you reach the Galactic HQ, you can pick up a TM for sludge bomb.
Gastly itself is awful early on, relying heavily on power-grinding or babying. Furthermore once fully evolved it is not effective enough to offset the cost of both obtaining, and running it.
8 levels of uselessness is definitely pretty bad by A-tier standards, but I don't think it's deal-breaking.
-It does not have the power to blow past most pokemon that it comes in against, and its lack of defence means it is often trading hits leaving it on 30%~ health after nabbing a 2HKO.
Attack power isn't a big problem for Gengar. It can 2HKO quite a bit of stuff if give a choice specs and taught the right moves (and isn't underleveled). Its low defenses does mean it's taking a lot of damage as it's battling and it finds difficulty sweeping opponents, but a Pokemon doesn't need to sweep everything in order to be A tier. When I play Pokemon games with teams of 4-5 Pokemon, each Pokemon only needs to be able to 1v1 1-2 opposing Pokemon per battle in order to win those battles. I think an important quality that sets A Pokemon apart from B Pokemon is that A Pokemon are more adaptable and is more likely able to 1v1 a given opponent, while using several B Pokemon might leave some important opponents "unable to be 1v1'd". Gengar usually does have this flexibility, especially when it has choice specs.
-It's coverage options, while powerful, are situational and limited. Thunder is nice against Wake, but after this their only real use is to hit Cynthias Milotic and Togekiss, both of which will heavily dent gengar if it misses. Thunderbolt is not strong enough against these two to warrent mentioning. Focus Blast is nice against Candice, although Gengars frailty + Hail means a miss can be fatal. Can nab a KO on Cynthias Lucario but again a miss is GG. Same story against Cyrus. Icy Wind while nice against the Chomp will not KO. Also pretty weak in general although the speed drop can be nice to be fair if you dont mind sacking/ reviving. Sludge Bomb was an ehhhhh STAB move that hit some things harder than shadow-ball, but still doesn't grab any significant KO's- gengar will only 2HKO and receive a hit in return dropping it below half HP.
You might be underselling focus blast (or I'm overselling it; at this point I'm not sure anymore). When combined with shadow ball, they form one of the few completely unresisted 2-move combinations. In addition to Candice and Cynthia, focus blast helps a lot against the many normal Pokemon and rock Pokemon throughout the whole game, as well as let Gengar sweep the steel gym. The accuracy is relatively annoying, but still soft-resetting for focus blast isn't too hard.
Gengars only real utility was against Lucian, and even then Bronzong was annoyingly bulky while doing massive damage back with Gyro/ Psychic. Mr.Mime was similar due to its high special defence.
I think Lucian is one of the worse fights for Gengar. It just can't seem to get enough OHKO's to avoid the super effective psychic attacks. I definitely disagree with the "only", as Gengar can help a lot against Volkner, Aaron, and Bertha just by spamming neutrally effective STAB attacks, and the choice specs make it significantly better in the Pokemon League. It can also use hypnosis to contribute to just about any battle (though hypnosis also has bad accuracy).
I too agree that Gastly isn't as good in-game as it is on paper. For the early-game there's not a lot of important uses for Gastly such as the gyms as it doesn't hit or tank a lot of mons early-game super effectively, even the Posion-types in the grass gym due to its frailness. Gastly also does horribly against the Commanders as Zubat's Bite already threatens it while Purugly is very bulky, resist Ghost-type moves, and also has Bite while Skuntank is a natural counter to Gastly and its evolutionary line. You'd think Haunter would do good in the Fighting-type gym but practically all the Machokes and Machops cary Foresight while going against Meditite is risky as some carry Confusion but some do not but Shadow Punch threatens it as well so its a basis of who can defeat each other first which isn't reliable.
Yes, Gastly sucks against Gardenia, Jupiter, and Fantina, but why are you even trying to use a Gastly in those battles? Jolteon and Scizor can be A tier despite completely missing those battles, so you can use Gastly much less painfully if you let it miss them, too. Why are you so worried about foresight when Gengar resists fighting even after being hit by foresight? Gengar's shadow ball one-shots Meditite, so that shouldn't be a problem, either.
As Haunter later on before it is traded, which the ideal time it should be around the 5th or 6th gym, it doesn't do as good because of that point in the game, there isn't a lot of things Haunter does well against. In Platinum, Haunter does well in the Ghost-type gym but has to be careful of faster Pokemon like Mismagius and higher leveled opposing Haunters but otherwise it does good. The emphasis on types Haunter is good at defeating is not really evident at the point where Haunter is present in the game (4th and 5th gym) therefore making it only useful against unsignificant route trainers that happen to carry Pokemon Haunter checks.
Why do you not evolve Haunter into Gengar immediately after evolving Gastly to Haunter? Why do you need to wait until after the 5th gym?
For Diamond and Pearl, a Gengar is needed for the Ghost-type gym if you don't have anything else to combat it, like in the case of Platinum and Haunter, your newly evolved Gengar should be higher leveled than the other Haunters and the Gengar to defeat them and be faster than the other Ghost-types and hopefully OHKO or else it could take severe super effective damage back and possibly not live due to its poor bulk. Again refrain from battling Drifblim and Mismagius, the former is rather bulky and can hit adequately hard while the other possibly KOs you easily but if you outspeed it you can take it out, as for the gym leader's Gengar, its up to who's faster. After the 6th gym Gengar's effectiveness starts to deteriorate as you get more involved with Team Galactic as they carry a lot of Pokemon that can take out Gengar pretty easily such as Stunky and Houndoom as Dark-types appear more late-game. Again, all the commanders can defeat Gengar with ease except for Bronsor and Kadabra (DP) which is practically Gengar bait. As for why Toxicroak beats you, Faint Attack. Cyrus is literally the worst thing to happen for Gengar carrying an array of Pokemon that carry Dark-type moves.
Gengar is bad against a lot of Team Galactic, but the 6th and 7th gyms have Pokemon that are weak to focus blast, so that's where Gengar can make up for its shortcomings. Mars's Purugly is also weak to focus blast.
There's not a lot of Pokemon Gengar can effectively target when you face your rival as its just an exchange of neutral attacks while the Normal-type Pokemon have powerful coverage while sadly the Poison typing of Gengar unluckily doesn't do super effective damage to Barry's two only Grass-types: Roserade and Torterra. As for the Elite Four, Gengar is really useful against the Psychic-type Elite Four as long as it can KO them without getting hit back as it can easily get revenge killed by them, as for the other elite four members, again, it is an exchange of neutral attacks. As for Cynthia, Gengar doesn't have any targets that are hit super effectively with STAB by it.
As I said earlier, Gengar can hit pretty hard and 1v1 a lot of things (including the rival's and Elite Four's Pokemon) using only neutral attacks. Its base special attack is one of the best in the game, and you do get a free choice specs. Aaron's Drapion doesn't know any dark attacks, which is convenient for Gengar.
On the other hand, Gengar is a nice Pokemon to have coverage TMs on meaning it's movepool is vast and can be used as great backup Pokemon, but for its STAB moves, it needs to KO with it or else it will end up being the one KOed, furthermore it is an adequate special attacker in terms of neutral moves. Overall Gengar is a really good special attacker and one of the most efficient Ghost-types, but its just that if Gengar and a Pokemon it can take on in-game start on equal footing, Gengar's poor bulk will let it down unless it completely walls the opponent which means Gengar is a great Pokemon to clean up weakened Pokemon usually without fail making this its primary function if you choose to run it on your team. Sadly, Gengar isn't as good in-game as it is competitiveness and therefore I too agree with it being moved to B.
I don't think Gengar's poor bulk lets it down that often. It can 2HKO many opponents, so it often wins unless it gets OHKO'd (or outsped). Gengar is definitely not weak enough to get OHKO'd by everything.