User blog comment:IamJakuhoRaikoben/Chapter 447: Escape Battle, Review./@comment-26451407-20150805081453/@comment-1153699-20150807000741

But when GMG Natsu sparred with him (chapter 417), Gildarts was still the same Gidarts.

Hiro used him as a benchmark for GMG Natsu's power compared to Tenrou Natsu's power (there was even a flashback). And using Gildarts as a benchmark for Natsu pretty much indicates Gildarts didn't change power-wise.

It was the same with Bluenote. Hiro didn't make Bluenote get stronger so we could see the results of Natsu's training.