You just debunked yourself there.
Gildarts strikes without magic are weak when compared to August, but with magic they are on that level, when Gildarts found out a way to get around August's Power Null, that's when he did damage.
False, read the fight again, Gildarts was getting dominated the entire fight even whenever he went for Hand to Hand combat which he should have had the edge in. August caught his punch and threw him aside like a child. Even whenever Gildarts did manage to damage August, August stood up and Gildarts was completely shocked that his attacked barely phased August. It's blatantly clear that Gildarts was getting his ass kicked and couldn't do anything to August, the difference between the two was massive and the only time Gildarts was able to damage August was by using his tool which couldn't even withstand the force of the attack.
calling his arm a tool is just saying that it isn't natural and it is only a an extension of himself, not to mention he survived several hits from August, therefore he was taking High 6-C AP, and doing just fine, Also he pushed back August on several occasions, there is nothing really complicated here, he damaged August on his own, so he should scale.
You're ignoring the context here, Gildarts calling his left arm isn't just him saying it's not natural. It's something that has special properties ( I.E being stronger than his normal attacks. ) and that's made evidently clear that August was way above Gildarts. Even with his tool, it was destroyed by the force of his attack and August stood right back up. Even in a 2v1 along with Gildarts being enraged he still couldn't defeat August. Also pushing August back doesn't equate to AP or durability. I could push someone who's physically much stronger than I am, being able to push someone doesn't mean you're as strong as them in the slightest. As for Gildarts taking hits from August I'd call that an outlier, considering later on August was able to pierce right through Gildarts with an attack that was meant for Cana. Again his Arm was also destroyed by attacking August. It goes against Newtons Third Law and is incredibly inconsistent. Also taking a attack and still fighting doesn't mean anything. For instance, Luffy took a kick from Kizaru and kept fighting, Ace got burnt by Akainu and jumped right back on his feet in the same panel. These things happen often in Manga so it's more of a stamina feat rather than a durability feat since Gildarts was heavily damaged by said attacks.
At best Gildarts would get "At least 6-C, possibly High 6-C with his mechanical arm." and that's still generous considering that the arm was destroyed by the force of the attack.