Gigi's poem is a prediction made by someone who feels unvalued and is ready to leave. The speaker imagines a future in which the person who took her for granted finally realizes what they lost. She pictures them wandering the streets, calling her name, wishing they had kissed her, loved her and kept her. But by then she will be gone without a trace.
The mood is a mix of hurt and defiance. There is a sting of revenge in the picture of her "on top of the world" while the other person is "stuck down under," yet the tone is also sad, because the whole fantasy comes from wanting to be wanted. The repeated "one day" and the list of verbs ("searching," "crying out," "asking") make the poem feel like a chant she tells herself for comfort. The last lines, with the stranger asking for "this girl's" number, are quietly painful: she has become someone the other person can no longer find.