I believe the situation with Gildarts is more of him finally going all out and a little of him advancing in strength during the time skip. I still believe Natsu is at most third in the guild, after Gildarts and Laxus. I doubt Mashima would simply discard the Power rankings in Fairy Tail. I recall that back when Natsu showed up after the time skip and before Laxus reappeared, many said Natsu is on a completely different level and that Laxus, Gildarts and no one else would come close to him and then Laxus showed up to prove himself. Gildarts was already showing insane feats in the past despite not going all out. Most reference his battle with bluenote and while I agree that he was pushed severely in that battle, it was mostly because he fought Bluenote on physical terms while under his gravity magic but made short work of him once he used his magic type. He probably didn't use it earlier because it's hard magic to control. Mashima also had to do that in order to prevent Gildarts from looking too OP when the manga was not ready for that. Right before the time skip, Natsu was still afraid of fighting Gildarts and the latter was able to send him flying while punching him jokingly. He got a little angry and basically destroyed an island. When he does all out, I believe it would be proven that he remains Fairy Tail's Ace. Laxus comes in next and probably Natsu after that. However, Natsu, Erza, Gray and probably Mira, gajeel are about the same level but if I was to choose, Natsu would be slightly above them.