Eh so Gilgamesh doesnt have Ten crowns?
"It's also another misunderstanding, since only Gil route was translated. You actually must read all 3 other routes for the full explanation. The irony is, all 4 servants misunderstood that power, until they actually got it, it's probably a 4th wall breaking joke Nasu did to reference the fans who got confused by his terms. It's a long explanation so bear with me.
The mystic code is actually called the "mystic code of Legend" (shinwareiso), it's basically "YOU the legend" given form. According to Rani it's a phenomenon given form. It's power is allow you to obtain the concept of YOUR origin. Where did you come from, What is your story? This is why such power is available to all things, even the weakest human can get this power. Nero had no idea what that was so she was confused, Nameless thought it's the Root, Gil thought it's his prototype and Tamamo thought it's her Golden Fox mode.
And...all of them were wrong lol. This is basically a conceptual armor (literally) against BB's power. BB's entire deal is that she is the earth mother, the origin of all life, thus all life on Earth cannot go against mama BB. However if you have the mystic code of Legend, you are wearing your own story as your origin. You are not originated from mama BB. Nero's story is of an emperor (her mystic code is called legendary mystic code of the emperor), she's the original fire. Nameless story is that he is a nameless hero forgotten by humanity (wrought iron hero legendary mystic code), Tamamo's story is the Golden Fox who represents the sun (legendary mystic code of the sun). Gil's story is that he's the king of heroes, the original hero, the one carrying power of the beginning (legendary mystic code of the beginning) of all heroes. That's how they cancelled BB's power. By flipping her own concept against her.
User of this mystic code, according to Tamamo, can "avoid the Third Eye power to manipulate events" that BB has. BB might not be able to tank Ea or Excalibur image or w.e with her body per se, but this Third Eye (Shiva intensifies) basically allows her to straight up discard the event of her getting hit, reversing all hypothetical damage, that's why you cannot beat her without the mystic code.
The mystic code also allows you to raise your servant stats to star rank to match BB. It doesn't give you heroic spirit or original power, it gives you more than that. Tho it couldn't give all of Tamamo 9 tails power. The downside of this, as Sakura and Rani said, it puts massive pressure on the saint graph so you can only use it once. Do it twice your servant blows up lol."
Well im also checked the scan in Gil route, its true that Gil only get a power in the same rank as BB but what kind of power we don't know, it never stated.
https://tsukinoura.wordpress.com/2017/03/27/entry-152-fight-to-the-end/
[The golden light fills up everything and Gilgamesh makes…an entrance.]
BB :: That power — It's the same rank as mine. — How wretched.
Gilgamesh :: Indeed. A king should never show his true form, even unto his death. However, my mongrel wagered her life to bring these rites to the attention of the crown. If I do not answer her call, who will. If the malice covering this place belongs to all the world, that's most convenient. Listen carefully, all beings. This woman is my contractor. Consider harming Hakuno Kishinami without my permission impossible from now unto eternity!
Hmm i Smell chaos.