Well what the gates does is allow a person to fully access all of their chakra at once, right? That is what the manga and databook say anyway.
So a ninja normally has access to about 20% of their total chakra, because the "chakra gates" limit the flow of chakra to the body, so the body doesn't wear itself out really fast.
Because of this the body is unaccustomed to using all of its chakra and it is really harmful to the user. This is why Guy trains his body only, because if he did not then the gates would hurt him even more.
So now is where I explain why the Guy with the gates can never be as fast as A or Bee or Naruto.
Normal taijutsu requires chakra (or part of the stuff that makes chakra) just like anything else. Normally taijutsu requires less chakra than ninjutsu and genjutsu. However when opening the gates Guy and Lee are enhancing their speed and strength with chakra.
Any ninja can do this, Sasuke did it in Part 1 to copy Lee's speed, which was necessary to perform chidori at that time. That is why it wore Sasuke out so much more than it did Lee, because Lee trained to be that fast, whereas Sasuke was using chakra to augment his natural speed.
So anyway back to the point. There are 8 gates and each gate allows for 10% more of your chakra to be utilized. You start out at 20% and by the eight gate you will be using 100% of your chakra. This is why opening the 8th gate guarantees death, chakra exhaustion is a sure thing if you are pouring out all your chakra. (This is another reason why ninjas have the gates too, to prevent death by chakra exhaustion)
So since Guy and Lee are augmenting their speed with chakra they will never be able to reach the speed of someone who has more natural speed than or someone who has far more chakra if that person is also using a speed boosting mode. (tailed beast mode, KM , lightning armor)
The reasoning behind it is this.
Let's consider A for a second. He has tailed beast level chakra yes?
Guy does not.
So let's say A has 5 times more chakra than Guy. And let's give them numerical values.
Guy's chakra levels = 1 (1/5th of A's)
A's chakra levels = 5
So normally each is using 20% of their total chakra levels right?
so that brings Guy down to 1/5th and A down to 1.
Now even if Guy were to open all 8 gates that would put his chakra he can access at any one time back at 1, which is now equal to A's levels.
Which means that they have the same chakra levels to put into augmenting their speed and strength. But A is not just augmenting his speed with chakra, he is augmenting his neuro-synapses with lightning (electricity is how are neurons fire) to increase his reaction times and speed even more than if he were just using plain chakra.
So if A were just using chakra to augment his speed, and he had 5 times more chakra than Guy (which is a conservative estimate judging by A's chakra levels and how fast Guy gets tired while in the gates) then that means they would be the exact same speed and it would matter who is faster without chakra augmentation. However A is using lighting as well to increase his synapses.
The same exact thing can be applied to Bee and Naruto, except Bee and Naruto have more chakra than A does. So that is why it is possible for Bee and Naruto to go faster than A, and why it is possible for A to move faster than Guy. (and especially Lee who can open less gates)
Hope this explanation makes sense to you. If it doesn't just tell me where I lost you and I will try to clarify it.
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