Well I am almost 100 percent positive that nowhere in the manga does it say that each gate increases the user's power by a factor of 10. That is ludicrous and sounds sort of like DBZ.
First off I will explain what the gates are and how they work. In the Narutoverse, ninjas use chakra for everything. They use it for taijutsu as well as ninjutsu and genjutsu, chakra is basically their version of DBZ's ki.
So ninjas have all of this chakra in their bodies, but to prevent them from using it all at once there are eight gates that limit chakra flow and strength to the muscles to prevent chakra exhaustion and the body wearing itself out.
What the gates do is release a gate on your chakra, allowing you access to more of your maximum potential chakra and strength (speed goes along with this as it is directly related to leg strength). So with each gate opened you have access to more of your maximum chakra, the gates does not increase the amount of chakra you have nor does it give you speed boosts like the raiton armor or sage mode, what it does instead is allows you to access your highest potential strength (and speed) and chakra output.
This however comes at a cost, with all of the chakra the ninja in the gates is using they become tired and lose chakra at a much higher rate. Also because their muscles are working so hard and at levels they don't normally the mucles start to tear and wear down. This is why Guy is always tired after using the gates and usually in pain depending on which gate he has opened, this is why he and Lee train purely in taijutsu and muscle strength, so that they can reduce the muscle wear and tear caused by the gates.
So now that we know what the gates are and what they do, let's get into why they could never be the best while used by someone who is not already the best.
Because the gates only allows the user to reach their maximum potential power and chakra levels, the gates user will always fall behind someone who was born with more chakra or has access to different modes that augment their own chakra. (Sage mode, Kyuubi chakra mode, raiton armor) Now this doesn't mean that those people are necessarily faster and stronger, it just depends on how much chakra they are putting into it, but it does mean that if those people are able to surpass the gates user.
My argument here is that you think Guy opening the eighth gate would become the strongest ninja in the Narutoverse, well that is literally an impossibility. He either doesn't have the chakra reserves or the chakra control necessary to push his strength and speed to levels given to people with bijuu level chakra levels.
As we saw in the fight against Kisame, Guy had to go seventh gate just to catch up to Kisame who was running away, and even then his attacks didn't even knock out Kisame let alone kill him. Kisame didn't move because he thought that his shark bullet would absorb Afternoon Tiger, so it was really Kisame's overconfidence that beat him. Guy's movements were still able to be seen, and his strength wasn't super tremendous as he smashed Kisame's body but Kisame was still conscious and later able to perform summonings and a water prison.
Back to the point, Guy and Lee can train their speed and strength, but you can only increase your chakra levels to a certain point, for the most part you are stuck with what you were born with.
So when A who also trains his speed and strength solely, uses his raiton armor there is almost no way that Guy can reach the speeds that A can, because a has bijuu like chakra levels. Maybe his strength is equal or above A's, because I am not sure if lightning armor increases strength or not, but either way he can't be as fast as him.
And as for Naruto and other jinchurriki, if you compare Guy using the gates to them, then that is just insane. What that is doing is saying that Guy's maximum strength and chakra levels is equal to or exceeds that of a bijuu and their jinchurriki's. Naruto's speed whilst using his KM mode is perhaps the fastest that anyone can get, except for Naruto who still has the means to go much faster as he has Sage Mode as well and does not have full access to the Kyuubi's chakra. (By the way can anyone imagine Naruto if he combined KM and SM and then used the gates? He has a high healing rate and near infinite chakra, so that would allow him 100% access to all the chakra he has, man that would be insane)
And I thought I read somewhere in this thread that the gates aren't used that often. Well that would also be incorrect as in either the first or second databook (made by Kishi so it is canon) it says that most jounin can open at least the first two gates. Kakashi used the first gate when climbing a rock wall, so in other words just for fun basically. The gates aren't something super rare and ultra powerful like being a jinchurriki or raiton armor, it is just that Guy and Lee have made it their focus so they have taken it further than most other people.
And to the people who say that it give powers equal or above that of a kage, what does that mean? What makes kages special? Usually it is their unique abilities that make them powerful, something the gates can't give someone. Most kages are faster and stronger than normal ninjas, but that is due to their high chakra levels (I defy you to find a kage who does not have high chakra levels), so that quote only applies to the physical aspect of being a kage. While a gates user who opened the eighth gate might become stronger and faster than most kages, they don't gain the special abilities of a kage and so can only reach their levels in regards to taijutsu but nothing else.
And before I go here is another example of the difference between the gates and someone born strong with high chakra levels. Guy has fought Kisame three times, and the second time Guy was forced to open the fifth gate to be able to fight on par with Kisame. But what do we know about that fight? It was a 30% chakra clone who didn't actually have the real Samehada. And Guy in the seventh gate was still not able to easily kill Kisame, who was taken by surprise and already weakened from before. If used by an average person the gates only puts people on par with exceptional people.
EDIT: to the Lord of Kaos, the third raikage would be able to withstand much more than a few hits from Guy's jutsus. That is if Guy is even able to perform more than one or two, he was exhausted after just running after Kisame and using Afternoon tiger once. The third raikage on the other hand survived the rasenshuriken, and more importantly is able to tank the TBB (as of now the most powerful attack there is), and we don't even know how many the third raikage took from the eight tails.
