So what, then, are we to make of this story of Gutei and his One-Finger Zen?
To arrive at an answer, we should go back to the beginning. Gutei attained enlightenment when Tenryu raised one finger. I don't know why Tenryu raised the one finger and the stories never tell. Perhaps he was raising it dramaturgically while making some point while speaking. Gesticulating. Perhaps he raised it to silence Gutei's questions and commentary. Perhaps he was pointing to the heavens, or gesturing "one" to indicate the unity of all things.
It doesn't matter. There was one person who attained enlightenment at the sound of a pebble hitting bamboo. The sound of the pebble didn't "mean" anything - it just was. There was another who attained enlightenment at the sight of cherry blossoms in a valley. The cherry blossoms weren't trying to communicate anything - they were just existing as they were.
Gutei attained enlightenment when Tenryu raised one finger. Because this was an intentional, human action, we want to identify some specific meaning to it, and Tenryu most likely had some reason for raising a finger, but it doesn't matter - the conditions were right for Gutei to awaken, and the finger just happened to be there.
From then on, Gutei used the raised finger to answer all questions. It wasn't that he had attached any meaning to the gesture other than it being a direct action - the expression of his teacher, which went beyond rational thought and meaning and was just a direct expression of things as they are. Suchness, if you will. Just be glad that Tenryu wasn't picking his nose when Gutei attained enlightenment.
Gutei's disciple later thought that the gesture had some specific meaning, and when asked what Gutei's teaching was, he mimicked the gesture. However, the poor fellow was mistaking the method for the lesson. Gutei's teaching wasn't the holding up of one finger - the gesture was just the action that's free from words, that's beyond this or that, that is simply suchness in the present moment.
Gutei disabused him of this delusion. It's upsetting to think that he literally cut off a finger. Maybe it was a figurative removal - the cutting away of delusion - not unlike Huike's supposed cutting off of his arm to show that he was free from grasping. But we should examine why a literal cutting upsets us so much. One finger is arguably a small price to pay for enlightenment, but perhaps we are still clinging to body-and-mind and are not yet ready to awaken. Dogen described awakening as body and mind dropping away, and if we are free of body and mind, we wouldn't much care about a finger.
The attendant attained enlightenment after Gutei had cut off the finger and then raised his own. But it wasn't these actions that caused the awakening in the attendant - once again, these actions, although extreme, don't "mean" anything in and of themselves other than whatever meaning the mind puts on it. As Zen Master Dogen explains, the attendant attained enlightenment due to years of practicing with his teacher. For him, the bamboo, the cherry blossoms, the one finger was this rather dramatic sequence of events.
The attendant attained enlightenment after Gutei had cut off the finger and then raised his own. But it wasn't these actions that caused the awakening in the attendant - once again, these actions, although extreme, don't "mean" anything in and of themselves other than whatever meaning the mind puts on it. As Zen Master Dogen explains, the attendant attained enlightenment due to years of practicing with his teacher. For him, the bamboo, the cherry blossoms, the one finger was this rather dramatic sequence of events.
But the cutting off of the finger is not the point of the story - it's just something that the mind can get fixated on. The point of the story is that in buddha-nature, human actions, just like the natural events, are beyond meaning or significance. The meaning or significance is later applied by the human mind and may differ from person to person.
After years of practice, Gutei observed Tenryu raise a finger, and at that moment awakened to his true nature. He observed this action-beyond-meaning his whole life, and thoroughly explored its potentials.
If you grasp for meaning, watch out!, because there's no account of Gutei's giving away his knife.