It is not that you should not trust but if you are looking for "how do I know I will not be betrayed" that is somewhat different. I trust Gilgamesh but I would not say I know every single part of him, especially now. I can say 'what we forged should not be broken, even by a fight to the death' but I cannot say that is how it is absolutely. Since an absolute would be a hundred of a hundred, a thousand of a thousand, until there was nothing more. I have not seen that far. But there is no reason to distrust in him either.