I have researched extensively, and I have not found the origin of the Eight Gates in Naruto, so I will assume it was never mentioned and create its origin story for plot reasons.
***
Tsunade was surprised to hear this, while Yujin was simply confused.
"Gate of Opening? What is that?"
Even as he asked, Yujin continued his work, not thinking it was relevant. Duy took a deep breath and explained for Yujin to understand.
"I don't have very large Chakra reserves, and that's one of the reasons why my potential as a ninja is low. However, I received a technique from the Third Hokage called the Eight Gates. It's a technique where I use my Chakra to open Eight Gates that exist in the human body; the more of these gates I open, the more potential is unlocked from my body, thus multiplying my strength several times. Gate of Opening is the first of the eight gates, and it's in the left hemisphere of the brain, where I am feeling the pressure right now."
Yujin found the conversation fascinating but didn't stop working, continuing his task. Later, he planned to explore these "Eight Gates" further. Tsunade, however, was lost in thought, and suddenly she realized something.
"Yujin, wait, maybe you shouldn't open Duy's first meridian..."
Before she could explain why, the pressure vanished, and Duy felt a good and relaxing sensation throughout his body. Like Tsunade, he began to feel the energy around him. It was as if he were experiencing the world for the first time. The unknown energy he had never felt before seemed to be everywhere.
"I hope I'm wrong..."
Yujin finally shifted his attention to Tsunade and saw the concern in her expression.
"What's the problem? Duy did it; now he can accumulate Qi too."
Tsunade thought for a moment and explained her concern.
"What if Duy's Eight Gates are the same as the Eight Extraordinary Meridians? The Eight Gates are the only technique Duy can use. What if opening the first meridian interferes with the Eight Gates?"
Duy was excited, immersed in the new sensations he was feeling, but he still heard Tsunade's words, and it surprised him.
"I'll test it."
Duy then focused his Chakra on the Gate of Opening, as he had done thousands of times before. This time, however, he felt enormous resistance. He spent several minutes in silence, trying to force the Chakra to pass through the first gate, but it was as if the gate were locked with a latch, preventing the entry of Chakra.
At that moment, he started to worry, and Yujin asked after seeing this.
"Are you sure these Eight Gates are the same as the Eight Extraordinary Meridians?"
Tsunade was distressed by the situation, and Duy was getting worried, but Yujin remained calm, analyzing the situation coolly. Duy began to explain about the Eight Gates.
He explained what the gates were, how to open them, the abilities of the gates, the consequences of opening them, and many other details. Duy was the greatest expert in the technique in the village, so he knew everything about it. Yujin, on the other hand, seemed relaxed and just listened silently.
"Hmm... What an intriguing technique. Listening to you, I'm sure that the 'Eight Gates' are the same as the eight block points of the Eight Extraordinary Meridians. The eight meridians do not refer to these 'gates' themselves but to the path that is made from one 'gate' to another. For example, from the first 'gate' to the second, it's called the First Extraordinary Meridian. It allows us to feel Natural Qi, increases our Qi accumulation speed, and gives us a greater affinity to Qi, providing better control."
"The Second Meridian is between the second and third 'gate,' and it also gives enormous advantages for martial arts training. It goes on like this until the Seventh Meridian, which is the path between the seventh and eighth 'gate.' It is speculated that only one person in history managed to open it; the God of Martial Arts... Recently, I just opened my Fifth Meridian, the path between the fifth and sixth 'gate.'"
Tsunade noticed something strange in his speech and quickly asked.
"But what about the Eighth Meridian? Is there a Ninth Gate?"
"No... The 'ninth gate' is the heart itself."
After a few seconds, realizing he didn't explain it properly, he continued.
"The Eighth Gate is superficially in the heart. It is theorized that if you can open the Eighth Gate, you can flood your heart with Qi through the Eighth Meridian and produce a different type of Qi. According to the theory, this different Qi would have the ability to give you eternal life, making you literally a god in human flesh. Whether it's true or not, I don't know. The God of Martial Arts I mentioned before didn't have eternal life, I think."
Everything about the God of Martial Arts was speculation, so Yujin didn't know what to say. Duy listened attentively, but he wanted to hear something else.
"What does all this mean? Is it impossible for me to reopen my gates?"
Feeling the anxiety in Duy's voice, Yujin felt a little guilty. He had inadvertently caused an optimistic and determined man to practically panic with his thoughtless actions. Yujin explained in a low, calm voice, trying to reassure Duy.
"I think whoever created the Eight Gates used the Eight Extraordinary Meridians as a basis, creating this forbidden technique. After all, if you can only open these gates temporarily and cause so much physical harm, with the opening of the Eighth Gate meaning death... It's a violent technique that doesn't consider the user, only the instant power gain. The Eight Extraordinary Meridians are the way we humans can acquire power; it's closely related to Qi training. Some call the Eight Extraordinary Meridians the Spiritual Root because of how important it is to us Martial Artists. When you open the first of the Eight Meridians, it never closes, and it brings extraordinary benefits."
"I think the reason you can't get the Chakra through the gate is that there's a Qi thread connecting the first to the second gate. If that's true, either we try to make it possible for both energies to flow through the First Meridian, or I erase the Qi thread. If we can't go with the first option and go with the second, it means I won't be able to teach you Qi. Without the Qi connecting each gate to the other, you won't be able to advance in your Qi training. Without the First Meridian, for example, you'll only be able to practice Qi for thirty minutes a day, and your control over the accumulated Qi will be nonexistent."
Duy fell silent upon hearing this, clenching his fists tightly. He wanted to learn Qi, but having to give up the Eight Gates that he had trained for so many years was not an option. Taking a deep breath, he bid farewell to Yujin, saying he would think about it, and left.
Tsunade did not expect this outcome for the first training session, but she could only accept it. Yujin, on the other hand, felt guilty. He should have explained more about the Eight Extraordinary Meridians. That way, the Eight Gates would have come up in the conversation, and they would have realized the problem.
He just hopes it's not irreversible.
***
Even without Duy, Yujin decided to continue teaching Tsunade. He explained again that to accumulate Qi, she needed to meditate using the breathing he taught.
"Do this as much as you can, but realize that our bodies have limits. If you don't feel an increase in your Qi reserves during breathing, it means you've reached your limit temporarily, so focus on something else."
Tsunade nodded, excited about the idea of a different training. She had trained in all the ninja techniques that interested her, so a new area, something completely different, a subject in which she was practically a pioneer in the Leaf Village, was very interesting to her.
"What do I do besides accumulating Qi?"
"Now, you need to accumulate Qi. Currently, the only Qi thread you have is connecting your First and Second Gates. How do you plan to train Qi if you have none to use? Keep that Qi thread there and just focus on creating more Qi. When you have some, practice your control over it. Move it through your meridians, including those containing Chakra."
Hearing this, Tsunade nodded and noticed something intriguing.
"Why did the Qi block the entry of Chakra into the first gate if the other meridians can pass both Chakra and Qi at the same time?"
This was something Yujin had already considered, so he explained.
"Unlike the other meridians, only the Eight Extraordinary Meridians have Qi blocking points... I'll call them gates from now on; I like the term. Perhaps because of these gates, only one energy can enter. Of course, we don't know, maybe it's possible for a second energy to pass; I'll test that later. The Eight Extraordinary Meridians are special in the human body, no wonder they are called extraordinary, so who knows under what kind of laws they operate. We need to study more about this."
Tsunade nodded and sat down to accumulate Qi. Yujin watched her for a moment to make sure she was doing it right, then decided to play with Chakra a bit.
He had Chakra, so in recent days, he had been playing with it a bit, practicing his control according to Tsunade's previous instructions. She had already explained how the initial Chakra training works, and he was willing to pay attention to it from time to time.
Currently, although Chakra seemed interesting, Yujin wasn't very interested in training it deeply. He had plenty of time, so he wasn't in a hurry. Learning to use Chakra wasn't going to immediately increase his strength.
Honestly, control over Chakra is similar and at the same time different from Qi. He had a more fluid control over Qi, perhaps because of years of training. His control over Chakra, however, was much more rudimentary and imprecise.
Today, he decided to train what Tsunade had talked to him about before: walking on walls.
Focusing Chakra on the soles of his feet, Yujin put his foot on the wall. He felt that the Chakra was trying to fill the space between the sole of his foot and the wall, so he focused on the sensation.
Slowly, he felt a bit of grip on the wall, as if he were merging with it, but he knew it wasn't enough, and he just kept trying to move the Chakra.
Time passed slowly as Yujin trained Chakra, and Tsunade accumulated Qi.