Sorry vyomov to burst your bubble but, it can't be both a powerful sweeper and wall Keldeo / Landorus. And it's probably more important to wall these two at this moment in the metagame.
Eh, it's weak to SR, it's base attack isn't great (125 isn't what it used to be), and most of the fighting types that it's meant to be walling can get past it just with raw power like Keldeo, Breloom, and Terrakion (well it's not meant to be walling Terrakion I guess, but you get my point). Honestly I've tried using it in this metagame to counter that ever common core, and on paper it seems like a good idea, but I just don't like it personally.
These calcs are using EVs on the bulky Dragon Dance set.
252 SpA Choice Specs Keldeo Hydro Pump vs. 88 HP / 0 SpD Gyarados: 146-172 (41.35 - 48.72%) -- guaranteed 2HKO after Stealth Rock
+1 252 Atk Life Orb Technician Breloom Bullet Seed (3 hits) vs. 88 HP / 4 Def Gyarados: 426-507 (120.67 - 143.62%) -- guaranteed OHKO
Firstly, if your trying to
wall the Keldeo + Landorus-I core using Gyarados, then you are running Rest Talk almost always. Heck, you're very likely running specially defensive EVs (yes this is viable on defensive Gyarados) in order to take the hits better. You cannot
wall Keldeo + Landorus-I using Sub DD Gyarados, only check. Note that this doesn't make Gyarados a bad pokemon.
If you are trying to sweep with Gyarados, then your calculations that you just posted are misleading in the extreme. Why the fuck am I bringing in my set up sweeper and trying to set up on a +2 Breloom (ignoring that Moxie is also common as fuck). Like seriously. Breloom cannot switch into Sub DD Gyarados regardless, since Gyarados can DD the switch (or Sub) and follow up with Bounce. GG
Secondly, again, why the fuck am I setting up on the most powerful version of Keldeo in the fucking game. Specs Keldeo nukes through fucking DD Dragonite, but this doesn't mean that DD Dragonite is shit. Again, Specs Keldeo loses in a practical sense (ie it doesn't counter) due to DD + Bounce. Scarf Keldeo cannot really revenge Gyarados either unless it packs HP Electric, and its still worried about Bounce.
Sub DD Gyarados might not "counter" Landorus-I + Keldeo, but it checks the core pretty damn hard, especially when the standard sets for the above mons are U-Turn / EP / FB / HP Ice and a Scarf set respectively.
Im sorry ShootinStarmie, but all im getting from your post is you expecting to run a sweeper while still "walling" Specs Keldeo (under Rain since thats what Gyarados is usually used with), which is pretty much one of the strongest pokemon in the metagame. Furthermore im seeing that argument that since this sweeper fails at walling Specs Keldeo (or a fucking +2 Breloom) it must be bad. Either provide better logic as to why Gyarados is terrible (hint, its actually not!) or refrain from posting as misleading and irrelevant calculations are just going to piss me off.
I apologize if I am being unfairly aggressive here, but 40+ hours without sleep means I have fucking zero tolerance for people (apparently) manipulating situations to make a pokemon look bad...especially when it actually isn't.
EDIT: On further reflection its possible that you meant that Gyarados cannot counter both Keldeo and Landorus-I with its DD set which is true, but it still pisses me off how your post implies DD Gyara should be taking Specs Keldeo (or that by not tanking it its shit) as thats plain wrong. Either way, can you be clear on what you expect Gyarados to do, since all im seeing is you expecting it to take on some of the strongest attacking sets in the metagame....while running a) not Rest Talk and b) a set which lacks a significant amount of bulk compared with other bulkier Gyarados sets.
tl;dr
Don't expect Sub DD Gyarados to counter Specs Keldeo and +2 Breloom because this Gyarados is a SWEEPER, not a wall. Try running a more bulky set (prolly Rest Talk / Chesto Rest) if you want to attempt beating the above mons.