The problem is Gengar are frankly shit. Gengar is worse than Lunala as a Ghost-type and worse than Eternatus as a Poison-type. It's bulk is absolutely horrid and leaves no room for gimmicks. It's weaker than Lunala and Eternatus. Gyarados also doesn't run Flying-type STAB. I did say there's no good Flying-type moves. Dragon Ascent was the best physical Flying-type move and Oblivion Wing was the best special Flying-type move.
The only meta Poison-types are Eternatus and Toxapex. Gengar (nor Toxtricity should anyone mention that) isn't used. The only meta Flying-types are Mandibuzz and to an extent Corviknight. While Gyarados does have Flying-type, it's mainly used as a Water-type setup mon, as Flying-type moves don't really hit anything useful.
To answer your question about specifically Belch Gengar. No, it doesn't work, because Gengar can't actually survive a hit from anything and Dynamax doesn't fare any better against it.
252 Atk Melmetal Anchor Shot vs. 252 HP / 252 Def Gengar: 153-180 (47.2 - 55.5%) -- 74.6% chance to 2HKO
252 Atk Zekrom Bolt Beak (85 BP) vs. 252 HP / 252 Def Gengar: 168-198 (51.8 - 61.1%) -- guaranteed 2HKO
252 Atk Zamazenta-Crowned Behemoth Bash (100 BP) vs. 252 HP / 252 Def Gengar: 177-208 (54.6 - 64.1%) -- guaranteed 2HKO
252 SpA Reshiram Blue Flare vs. 252 HP / 252 SpD Gengar: 223-264 (68.8 - 81.4%) -- guaranteed 2HKO
Gengar is 2HKOed by basically anything. OHKOed by a lot more. I'm usually a fan of gimmicks. But I don't see this working.
So these are the Calcs after the +2 Defense?
My immediate thought is: Stuff Cheeks, which triggers +2 Defense, +1 Special Attack from Petaya Berry, and x2 Speed from Unburden. Then next turn use Gigantamax to Trap, or boost SpA, etc.
Here:
252 Atk Melmetal Anchor Shot vs. +2 252 HP / 252 Def Gengar: 76-91 (23.4 - 28%) -- 89.5% chance to 4HKO
252+ Atk Teravolt Zekrom Bolt Beak (85 BP) vs. +2 252 HP / 252 Def Gengar: 93-109 (28.7 - 33.6%) -- 0.3% chance to 3HKO
252 Atk Zamazenta-Crowned Behemoth Bash (100 BP) vs. +2 252 HP / 252 Def Gengar: 88-105 (27.1 - 32.4%) -- guaranteed 4HKO
The Behemoth Bash does the same after G-Maxing since it doubles in power, while Gengar doubles in HP.
This is assuming I get hit on the turn I use Stuff Cheeks.
Now, if they send in the Pokemon on the turn I use Stuff Cheeks, I can Gigantamax for some extra survival to cut that damage in half again, except Vs Behemoth Bash, and even cut the damage in half Vs Reshiram.
The goal is to Gigantamax for the extra trapping and boosting.
But how much is Gengar dealing back? Even without Gigantamax
+1 252+ SpA Gengar Blue Flare vs. 252 HP / 252 SpD Melmetal: 480-566 (101.2 - 119.4%) -- guaranteed OHKO
+1 252+ SpA Gengar Blue Flare vs. 252 HP / 252 SpD Zamazenta-Crowned: 282-334 (72.6 - 86%) -- guaranteed 2HKO
(Modest is for max damage with Unburden for Speed)
+1 252+ SpA Gengar Belch vs. 252 HP / 252 SpD Zekrom: 255-301 (63.1 - 74.5%) -- guaranteed 2HKO
+1 252+ SpA Gengar Belch vs. 252 HP / 252 SpD Reshiram: 225-265 (55.6 - 65.5%) -- guaranteed 2HKO
If I Gigantamax:
Once the Sun is set, next turn I actually will KO Zamazenta if it stays in.
Even if Melmetal uses Haze, the sun will set and be the equivalent 50% boost for a solid 1HKO.
Against Reshiram, and Zekrom I would get up to three turns of +1 SpA off of Max-Poison and continue to sweep, now at +4 SpA, or x3 (including Petaya Berry).
Gengar is not frail, it has double defense and double HP, and gets stronger under the sun, and/or accumulates additional +1 SpA boosts. It has double Speed to KO Imposter without fear as well.
Belch is 180 base power after STAB and keeps SpA sponges like RegenVest Primarina from switching in.
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For Gyarados, Brave Bird is still available for Magic Guard Life Orb sets, letting it pack Shift Gear/Bolt Beak, Fishous Rend, Brave Bird, High Jump Kick, removing Stealth Rocks, and other passive damage, and keeping Pokemon like Volt Absorb Golisipod in check.
HJK is a 2HKO even on Prankster Haze Melmetal (which cuts Fishous Rend in half by going first), and doesn’t fear Flash Fire like V-Create does (which also gets cut in half by Rain from Fishous Rend).
252+ Atk Life Orb Gyarados High Jump Kick vs. 252 HP / 252 Def Melmetal: 242-286 (51 - 60.3%) -- guaranteed 2HKO
Magic Guard also prevents Life Orb damage during Dynamax, so you can bluff Choice Band since the foe doesn’t see recoil.
Lastly Max-Fighting accumulates +1 Attack boosts so it can switch off with Max-Flying for Speed and Attack power, not using a moveslot for Shift Gear is, now optional.