A lean form but with higher muscle-bone-nerve density than G4, signifying rubber's efficiency of creating internal compression of Luffy's body structures. Compressed muscle and bone means much harder physical defense and attacks even compared to G4, denser nerve means much faster reaction and all aspects of speed.
Luffy achieve this lean form by fusing his rubber with CoC Coating. So i imagine an effect like a lightning-surfaced ball of energy zone made of AdvCoC constantly surrounding (fused with) Luffy's body. Providing impenetrable defense, and whenever Luffy wants to attack then the energy will shapeshift and release AdvCoC laserbeam in every punch or kick, shapeshift into a spiral-form if Luffy releases a Gomu2 Rifle, unleash a rain of lasers in Gomu2 no Storm, etc.