The Eight Inner Gates are eight specific tenketsu in the user’s chakra system. The Eight Tenketsu regulate the flow of chakra within a person’s body. They do make a person weaker, but also ensure that they don’t die. After going through rigorous training, 5 Shinobi have gained the ability to control the flow of chakra through these Tenketsu at will.
Shinno was a Shinobi from the Land of the Sky. He was the main antagonist in the movie Naruto Shippuden: Bonds. He was in possession of the Dark Chakra of the Zero Tails. Under the influence of this chakra, Shinno was able to mould chakra without any effort and his body went way beyond human limitations.
He claimed to be able to access all 8 Inner Gates without any drawbacks, that is, if he possessed the Dark Chakra. He would’ve been a cool villain, if only he were cannon.