Does Gyarados or Suicune suck because they don't have water gun?

Not at all.

Gyarados is still potent with even just Hydro Pump, and finds a decent amount of use with Dark or Dragon moves as well. Would he be better with a Water quick move though? Sure.

Suicune sucks because of his stats. Even with Water Gun, Vaporeon would outclass him by a wide margin.

Yes, their merely purpose should be as water specialist/ tanky attacker, but that isn’t the case

Pokemon go's stat / cp formula favors vaporeon more then suicune. Thats why it sucks. Suicune's stat total is higher but still bad with this formula. And yea water gun could help a bit but still worse then vaporeon.

Gyarados is another story tho. With water gun gyarados outclasses vaporeon on attack. Not even close. But sadly gyarados cant learn water gun. We are hoping for waterfall...

Yes, but Water Gun also sucks. One benefit of a new water fast move would be to make it stronger than Water Gun to give some other water pokemon a step up in comparison to Vaporeon.

wrong, its just vaporeon makes them suck