i thought the 8 gates was ninjutsu
Not really, it isn't a mode like raiton armor or Sage mode.
In the Naruto world, there are chakra "gates" inside of a ninja's chakra pathway. These gates are designed to limit the overall flow of chakra, so that the body does not get damaged from so much power flowing through it at once.
When one opens the gates, what they are doing is releasing some of those barriers so that they have access to a higher percentage of their chakra (kind of like Frieza in DBZ). So the more they open, the more chakra they can use at once. So the what the gates ultimately leads to is someone using their max chakra potential, and maximum potential speed and strength.
So it isn't technically a ninjutsu because all they are doing is allowing themselves to use all of their power in a short span.
That is why the gates will always fall short when compared to someone with monstrous chakra. Let's say that Kisame has 5 times as much chakra as Guy (which in my opinion is selling Kisame short). So normally a ninja uses 20% of their power according to the manga. So kisame has a chakra level of 1 because he is using 1/5th of his chakra levels.
Guy is at 1/5th to begin with since he started at 1. When he opens the gates he adds a percentage back every time until he gets to his maximum chakra potential, which is 1. So even with all the Gates open Kisame and Guy would be using the exact same amount of chakra, but Kisame still has all of that chakra potential left aftwerwards.
This matters because chakra is used in everything in the Naruto world, it is used in taijutsu, ninjutsu, and genjutsu, although taijutsu requires less. But when someone augments their speed and strength with chakra (Guy does this in the gates, Sasuke does this for speed, and Tsunade does it for strength) then it requires more chakra than normal taijutsu would allow.
So while Guy could become as fast and strong as someone like Kisame, the person with more chakra will always have the potential to be faster and stronger. So power in Naruto sort of depends on determination and willpower, but it also depends on if you were born with large amounts of chakra.
It makes you think though, what about those chakra beasts like Naruto and Kisame? What if they opened the gates and were able to use more of their chakra potential?
P.S. Sorry if you didn't need all of that, the answer is that it isn't a ninjutsu. I just felt like explaining the gates and how they work.
