Yea the other slayers also have dragons coming out too, but only saying that there is more to Natsu's improvement than training. You can even put him in a unique category because he is an etherious.
Gildarts got beaten by Acnologia in an instant, it says more about Acnologia than Gildarts. Surviving that is very worthy I think.
The fight vs Bluenote is basically showing that in the end Gildarts can beat Bluenote with a big spell. He doesn't go all out with his magic, just uses a powerful attack. I've always thought Gildarts couldn't do a mountain punch because it was Fairy Tail holy land Tenrou Island and he doesn't want to destroy it. Natsu required similar or less effort which is why I put Natsu above.
As for Gildarts not going full power I buy that, but in the same token Laxus and Natsu have not gone all out either. Gildarts mountain punch was a serious attack, maybe at the level of Natsu's demolition fist. Laxus' attack was less serious than both, but likely the most impressive.