You're comparing speed of punches to reflexes? Gildarts could be a quick puncher, but that doesn't mean his reflexes could be the same. Plus, it's not like August expected Brandish to stab him, so how could he react? Not hard to believe he didn't react because he didn't expect to be betrayed by someone he saw as his granddaughter. Or, he likely chose not to react because he figured the dagger wouldn't do much damage to him.Gildarts has casual mountain busting punches. He has thrown 13 punches instantaneously and no one can react to poof! speed.
If you combine those two, it means Gildarts can throw several mountain busting punches instantaneously. When Brandish brought out the knife he saw it but didn't react fast enough before getting stabbed.
Gildarts can blitz his opponents with Empryean, a mountain busting punch. That attack used against Bluenote had so much power in it that he sent so much force upward, the shockwave from that divided the clouds. Clouds are multiple kilometers in the air and Bluenote was gone by the time rustyrose hit the ground. If one of those punches landed on August he would be demolished.
When Serena used his Dragon roar, Gildarts destroyed the spell instantaneously. No one even noticed his arrival until the aftermath of the attack. That's a speed feat within itself.
Bluenote lost to current Natsu, who was overwhelmed by August. Unless Gildarts got stronger, nothing implies he's better than August. And Gildarts himself said Serena wasn't even close to his potential as a zombie. I"m sure it's hard to notice someone's arrival when you're worried about a powerful attack wiping you out, though.
