In just three lines, the speaker describes a shift in how they understand someone. "I once knew a girl" sounds like looking back at a person from the past. Then comes "I know him better now," and the surprise is that it seems to be the same person. The poem doesn't spell this out, but the title, "Gender," points that way: the speaker once knew someone as a girl and now knows that person as a man, and the knowing is deeper for it.
The speaker's tone is calm and accepting rather than angry or confused. There is no fuss, only a quiet statement of fact. The last line, "The winds of trust part all clouds," uses weather as an image. Clouds suggest doubt, confusion or things hidden, and trust is the wind that clears them away. The mood is gentle and hopeful, with a sense of relief. Understanding doesn't come from the label but from honesty and trust between two people.