Garp seems to have used Hardening (Obv) + Barrier CoA (For Range) + Internal Destruction CoA (For Maximum Effectiveness) + CoC Coating.
Shanks also done the same, He clearly used Hardening & CoC Coating, while Internal Destruction was added to make Kid's Weapon Explode & Barrier was also needed to Shield himself
These Two Attacks coming Back to Back, being Very Similar & also Shanks Attack is same as Roger's who was Garp's Rival
Clearly suggests that Galaxy Impact = Divine Departure
And for those who haven't figured it out yet, Chinjao's Head was clearly as Strong as a Meito Sword
So their Clash was to Indirectly tell us that Garp's Punch is as Strong as a Supreme Grade Blade
He clearly was Roger's Level & a Brawler Version of him
Also seeing how Shanks & Roger have Same Signature Move
I wonder if Luffy's Final Version of Gomu Gomu No Pistole is gonna be Galaxy Impact or similar Name
(After all, He added Jet, then Hawk, then Roc, so it's Upgrades are clearly being represented with Sky-Based Names, so Galaxy fits)