Good point, although we saw how the mountain crumbled just from Doffy leaving the whole that Luffy left him in, so I doubt it would last from hundreds of hits from a Kong gattling...
Must admit I can't even remember the moment you're talking about... Don't know why but TB is quite hazy to me which is why I'm going over it again, up to the SH's stalling / taking on Oars at the minutes. But even still, an on the floor Doffy with a Luffy just hovering over him at close range to deliver a gattling, I completely can't even imagine it, I know it's his signature move and all but with the way G4 works, I don't see a gattling really happening, especially a Kong one.
******
As this gent quite rightly put, he did go flying onto the deck of a battleship. Plus, the anime over exaggerates this moment, the manga show's Luffy remembering people saying stuff and I would of thought this amounted to 2 seconds worth of hits before Lucci started being knocked back. Plus, the reason why Lucci stayed on the spot for so long was probably due to his Tekkai, it endured Luffy's attack for mere moments.
Now you may say Doffy will do the same but with Haki, however Luffy has already demonstrated that G4 smashed his haki and then some, so he would have to keep up with a flying Doffy with close quarters combat of continuous hits.
PLUS PLUS a huge reason why I don't see Luffy doing a G4 gattling is that he can't even keep himself still for it, so an aerial one is actually more than likely the way he would have to use it.
Regardless, my opinion is still that I can't see him having a G4 gattling, nor that he will go back into G4 to finish Doffy. I just don't think it would be best for the story but that's not for me to decide... Unfortunately