Gaban states the reason they couldn’t beat Harald is because of his CoC, which has nothing to do with them not knowing how to cancel his immortality. Lol
idk why you are lying and trying to call me illiterate
We literally see the aftermath and Gaban only mentions that Harald was unstotabble because they couldn't destroy his regeneration (a.k.a they didn't know about metal heart).
Gaban doesn't call Harald strong and unstoppable because of his strength. He only wanked his regen.
This situation was literally whole big misunderstanding where they thought that maybe Loki's DF had the ability to turn off immortality, which was false.
Let's also not forget that Shanks and Gaban wanted to save Harald, which is why they placed hopes in Loki with his new DF maybe able to turn Harald back (he couldn't)