Gildarts would pummel OS on his own. Only person he'd have to worry about is Midnight, exactly like Jellal had to.Tbh, Jellal showed me way more impressive things than Gildarts did. He took down the OS on his own. Even though any Spriggan (maybe Ajeel not) could do this as well, I can't see Gildarts do it like Jellal did.
Sema may be the strongest caster spell we've seen so far. Gildarts didn't show me anything as impressive as Sema.
Both Laxus and Jellal are > pre-skip Gildarts.
As far as Gildarts impressiveness goes he actually can lay claim to beating someone of Jellal's caliber in Bluenote. His physical prowess is of the highest tier preskip excluding dragons. I rate Empyrean very highly pre-skip, it essentially can take out Bluenote tiers. Gildarts mountain punch is one of the best feats pre-skip. I don't think it's as powerful as Empyrean, but it cracked a whole heap of rock.
Sema is a very impressive spell but you have to remember it took out people of Cobra's level. Cobra was still able to move albeit he was in no condition to fight. Now there is no way I can grant Sema the capability of taking out Gildarts because it took out Cobra. There is a vast difference between the 2. As far as human wizards are concerned in pre-skip, on Hades can make the claim of having better physical durability and endurance than Gildarts - and Hades vitality is enhanced by demon heart. The best thing about Sema is that it's basically unstoppable, while Gildarts' Empyrean can be dodged.
I'd say Gildarts beats Jellal, high difficulty, or maybe med-high.
I place Laxus above the likes of God Serena, Ajeel, Wahl, DiMaria and Jacob; but below August. A nerfed Laxus is essentially Spriggan tier. Jellal I predict to be somewhere around Laxus level.I'd say Laxus and Jellal are equal. Both of them are more or less Spriggan tier and I can't see them beating a Spriggan without having a really hard time. There are still a view Spriggans who are stronger than them. I'd say they are equal with mid tier Spriggans.
Base Natsu is below Laxus, Jellal and Gildarts.
It's up to Gildarts to prove if he is above Natsu's level. Natsu can fight quite well, as seen v Bluenote. And that was not a FDK spell. Gildarts needs to show he's got that level of game. I don't by into the premise that Gildarts was holding back vs Bluenote, I do believe that he fought seriously; otherwise that would be bad writing by Mashima. So, Gildarts pre-skip is somewhere around GoI/Bluenote level in this timeskip, and hence would need to improve to get to Spriggan level.