I think this is an interesting discussion, but most of the responses have been lackluster at best. I'll attempt to do better. I use Swampert on a good number of my teams, but rarely do I find myself considering Gastrodon. Why?
Well, simply, Gastrodon cannot do what I rely on Swampert to do. I rely on Swampert to switch into dangerous Pokemon like Tyranitar and be able to deal with more or less every set that they usually run. Roar is a big part of this. Yawn simply does not cut it when you realize the majority of set-up threats in the game either hide behind Substitutes or are Sleep Talkers anyhow. Roar not only deals with these threats in a straightforward manner, but you can really rack up a lot of residual damage if your opponent can't threaten you directly. As for other utility options, Stealth Rock is a huge asset for teams that have no better candidate for it. Not that Swampert isn't a great SR user!
Let's not miss anything in the calculations here, though. Comparing the EV spreads listed in the OP, Swampert still takes everything beside Grass Knot and Low Kick better than Gastrodon can. Generally, Recover more than covers for the difference in tanking ability, but it's worth mentioning that Gastrodon does indeed take hits on both sides worse than Swampert does and can be 2HKOed in situations Swampert would not.
However, what really takes the cake here, is what Swampert can do after taking the hits you need it to. Roaring around their team has already been mentioned, but is really a huge asset when you realize how few things want to stay in on Swampert. Using the same EV spreads (The Mixpert one I use regularly), Swampert is firing off EQs from 256 Attack, vs Gastrodon's 203. That's quite a difference, as has already been explained. The real kicker for me is that Swampert learns Hydro Pump. Hydro Pump off 219 SpAtk obviously outdamages Surf from Gastrodon's 220, but even if you're using Surf the damage difference is negligible. Hydro Pump allows Swampert to severely punish physical tanks who want to come in and set up, though. Torrented Hydro Pump is scary even from a mediocre SpAtk stat, and things like Gengar risk an OHKO if they bring you below the threshold with an attack. And yes, the payoff is definitely worth the miss chance.
I definitely agree with the mindset aspect as well. Swampert is a bit more threatening in all aspects. You know you're never getting in for free because of a Recover, and Swampert is much more likely to attack as a result of that. You also will likely at some point have to play the dreadful 'Will it attack or Roar?' game, wherein you want to damage it so it stops roaring, but don't want to risk losing something if it does attack. Gastrodon just doesn't evoke anywhere near the same response, and it's not just because it's sprite is a lot less threatening!
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Cute but slimy
<---- Clearly insane and probably going to kill you
Man, that wall-eyed look gets me every time.