If a Gastly is spamming Sludge Bomb, that just makes it more easy to counter. Sludge Bomb is an awful move to spam due to its awful coverage. Ghost is much easier, and far better, to spam due to its neutral coverage.
Gastly is by no means more dangerous than Misdreavus. Gastly doesn't have the fearful 19 Spe like Misdreavus has, and is thus outsped by top-tier threats including Misdreavus herself and Murkrow. Many special walls including Lickitung and Munchlax, without SR, easily counter Gastly just as easily as they would Misdreavus. However, Misdreavus can neuter both of these switchins with Will-o-Wisp, something Gastly can't due to its lack of any sort of bulk. Gastly also doesn't have Nasty Plot, or the ability to run several sets that each function extremely differently, like Misdreavus can. I see what you mean about them being different, to an extent, but making it out to seem like Gastly is more fearsome in this metagame than Misdreavus is, is absolutely absurd.
Whatever outspeeds Gastly in the 19 - 20 Speed tier cannot switch into Gastly reliably:
196 SpA Life Orb Gastly Sludge Bomb vs. 0 HP / 0 SpD Eviolite Murkrow: 17-21 (77.2 - 95.4%) -- guaranteed OHKO after Stealth Rock
196 SpA Life Orb Gastly Sludge Bomb vs. 0 HP / 76 SpD Eviolite Ponyta: 12-16 (57.1 - 76.1%) -- 6.3% chance to OHKO after Stealth Rock (not sure if this the standard spread but it certainly cannot come in reliably)
196 SpA Life Orb Gastly Sludge Bomb vs. 0 HP / 0 SpD Elekid: 21-25 (100 - 119%) -- guaranteed OHKO
196 SpA Life Orb Gastly Shadow Ball vs. 36 HP / 0 SpD Eviolite Misdreavus: 18-26 (78.2 - 113%) -- 93.8% chance to OHKO after Stealth Rock
196 SpA Life Orb Gastly Shadow Ball vs. 36 HP / 0 SpD Diglett: 21-25 (116.6 - 138.8%) -- guaranteed OHKO
196 SpA Life Orb Gastly Sludge Bomb vs. 36 HP / 0 SpD Eviolite Staryu: 13-17 (65 - 85%) -- guaranteed 2HKO after Stealth Rock
How does Lickitung counter Gastly? It does not, it is a check, but a very shaky one as the chance to 2HKO is 11.3% combined with a chance of 30% of Poison from Sludge Bomb mean it can be 2HKO'd about 20% of the time. Lickitung can also do literally nothing to Disable Gastly. Misdreavus will NEVER break through a full health Lickitung even with Will-o-Wisp; a Burned Knock Off or Shadow Ball from Lickitung will still beat Misdreavus.
The same goes for Munchlax;it also cannot touch Disable Gastly either.
I consider it more dangerous because it has far fewer checks: it has Pawniard and Stunky, and to some extent Sandile but it loses to Choice Scarf Gastly. Misdreavus has many more: Porygon, Munchlax, Pawniard, Stunky, Lickitung, Houndour, Murkrow, Litleo, Carvanha, etc.
The strength from LO Gastly comes in the form of Subsititute; If it forces a switch (which is not hard to do at all) You must stomach an attack from 19 SpATK boosted by Life Orb. This is not the case with most Misdreavus, as they must take a turn to boost and generally do not give up Will-o-Wisp for Substitute. Even if they do, 236 SpATK Shadow Ball from Eviolite Misdreavus is no where near the power of Life Orb 196 Shadow Ball / Sludge Bomb from Gastly. In fact, Gastly hits only slightly less hard as a +2 Misdreavus with no set up.