SECOND GEAR Red hawk
Riffle
Bazzuka
Cannon
Gattling Gun
First lets start off with luffy's basic punch.
Its what he used to hit vivi when there were in a brawl.
But from that we have to different versions of hits. A long range punch in which luffy slings his arm forward like the way a gun slings a bullet (Luffy being the gun and his fist being the Bullet making luffy a Pistol) And luffy calls that gomu gomu pistol.
A stronger close range attack where luffy runs towards his enemy leaving his fist stationary (In this one His fist is the gun and he is the bullet flying at his enemy) And then snapping his arm hard into them.
These are the basics of his punches. Ofc there are stronger versions of attacks and kicks and Special techniques he came up with but for arguments sake right now we are gonna ignore them
Any thing that is up close is a bullet. Anything far away is a pistol. Jet Gattling or any gattling is an advanced pistol shot or gomu gomu shotgun, in which luffy swings his arm and it appears he has multiple arms striking dozens of times. And Gomu gomu shotgun is just a vibrating pistol. So gattling is still a pistol. So for anything to have a GATTLING form it has to be long range and a descendant of pistol
Red hawk is a jet bullet. Still a principal bullet where luffy runs at them and super heats his arm then rips it into his enemy up close. For something to have a RED HAWK form it has to be up close and a descendant of bullet.
Now all that Aside Theoretically luffy's strongest attack isn't red hawk or grizzly magnum or Elephant gun. Its something way older and way stronger.
Storm. Gomu gomu storm was a strong enough attack for alabasta luffy to wreck an entire thousand year old temple. Luffy only destroy's something so big again at enies lobby. But it took Enies lobby luffy Second gear and third gear to do what he did to the first prop the same thing storm to that temple did without difficulty.
So Storm and gattling were both used to cause large amounts of damage. BUT Luffy needed Gatling + Second gear to do what base Storm did.
Now lets talk about how luffy if in the air can stir up lightning in tandem with any of his blows. As shown when he fought chinjao. And for those who think that was haki it was confirmed in the anime to be lightning.
That being said in raw destructive power theoretically luffys strongest attack would be
Giant Elephant Jet Thor Storm.
Big haki covered gear third fists. Leaping into the air while in gear 2nd as well. Charging up his fist with lightning filling up with air and twisting up into a tight coil. Releasing thousands of super fast super heavy lightning charged elephat guns to indiscriminately level what he pleases.