The speaker watches a laborer and sees something quietly astonishing in him. The "miracle" is that he takes in every threat and every promise, all the pressures and hopes that hang over a working life, and swallows them down into his belly. He does it so smoothly that it looks ordinary, which is part of the wonder.
The last three lines shift the picture. What the laborer holds inside becomes "a better world," one that is "so truly round" and does "so little hurt to any man in it." Roundness suggests wholeness, something without sharp edges or corners to snag on. The poem seems to say that the man has digested the world's dangers and turned them into something gentler.
The speaker's tone is admiring, even reverent, but calm. Nothing is shouted. The mood is hushed and observant, a kind of respect for a person who absorbs trouble without fuss. The word "gut" in the title ties this to instinct and endurance.