No, you are mistaking DC for AP. You're herp derp DBZ scaling atp, looking for the biggest booms. You're better than that.
Roger and Garp's final attack is far superior to any attack we've seen in One Piece thus far, yet it had no DC. Luffy's attack that broke through Imu's shield had no DC yet it's seemingly the strongest attack Imu's been hit with thus far.
Gaban's attack wasn't as flashy as Loki's but he made up for it in attack potency. Hiding behind lethality here is a cop-out.
Your Asura vs Luffy's punches comparison is a false equivalency because Gaban didn't scar Imu. His haki strike alone left Imu bloodied and then his attack left Imu genuinely floored and impressed, triggering Imu's transformation.
You could argue lethality if Gaban say chopped off a limb, or scarred Imu etc... but he didn't. The damage was portrayed as generic, Imu just laid there floored in the same way he laid in the Adam tree after Loki's attack except with more damage from Gaban.