Why does everyone underestimate Gai sensei's ability? Think about the quantum leap from Asa Kujaku to Hirudora; it went from being a football field sized flurry of fireballs to an island sized shockwave that nearly knocked Naruto off his feet, miles away (from just a punch!) It messed up Madara for a chapter. Imagine what kind of power jump the 8th gate entails? Especially considering he'll have no reason to hold back. He'll be able to reach the power limit of every cell in his body.
Utilizing chakra the natural way, via normal cell respiration is the most efficient way to produce power. Any other method (e.g. infusing it externally to form jutsu), just doesn't compare. It's the inefficient, safe way of doing things. Just wasteful, and Gai knows this.
When Gai releases the last gate, located at the heart, it'll be like NOTHING we've ever seen before. All the other gates up until this one are child's play.