That argument doesn't work because your argument is in effect "just plug in this pokemon to deal with" when suspect testing doesn't care about whether a person can deal with a particular pokemon but rather does it alter the metagame in a negative fashion. And the answer is yes.
Look at all those pokemon you named and this is barring the particular Greninja you are facing doesn't have the move to take you out. The fact that you have to pick and choose from all those sets of pokemon and arrange in them in a way to counter 1 pokemon tells us all we need to know.
All Greninja has to do is surprise you just once with a move and you are in really big trouble. Even then you won't know all its movesets and so you have keep on scouting before you can properly handle it. Then after handling it and smiling to yourself, you realize that you have to face 5 other really strong OU pokemon to win the match.
It is a menace to the tier and predicting/scouting/running odd EV spreads just to check this frog is a pain in the ass to the entire OU metagame. Ban him.
"Tells you all that you need to know"? Firstly, name a single pokemon that i listed that isn't viable in OU for other purposes (chansey is your best bet, but still). And furthermore, those were counters, not checks (almost all of that list was checks even with Greninja running the right move anyways). If you don't have ANY of those on a team, you can still dish out a lot of damage to it with either priority moves or a choice scarf.
He said it hard counters Greninja, and I said that it only checks it (And it can't evens switch into Gunk Shot) not that Greninja can always take out Gyarados
Here are some calcs without Stealth Rock
Moxie:
40 Atk Life Orb Protean Greninja Gunk Shot vs. 0 HP / 4 Def Gyarados: 203-239 (61.3 - 72.2%) -- guaranteed 2HKO
Intimidate:
-1 40 Atk Life Orb Protean Greninja Gunk Shot vs. 0 HP / 4 Def Gyarados: 136-161 (41 - 48.6%) -- guaranteed 3HKO
(Keep in mind Gunk Shot's 30% chance to poison)
OK, i misunderstood you. I thought you were implying that it doesn't check it. Anyway, Gyarados will be almost exclusively be running the intimidate set, as intimidate is way better than moxie. Using your calculations, it can still do the Ddance and murder it. However, you forgot the main thing that makes gyarados not a counter to greninja: ice beam.