Whom is the poet Wordsworth addressing when he writes "stop here or gently pass," in the poem "The Solitary Reaper"?

This is one of Wordsworth's famous poems that describes
the sight of a female reaper reaping in fields. Wordsworth is so entranced by the sight
that he stops and stares and reflects on the sight. The words that your question refer
to come in the first stanza, and presumably are mentioned to any who are with him or who
may be passing on the same path. Note their meaning: Wordsworth is so wrapped up in the
vision of the reaper and her singing that he doesn't want any interruption to alert her
to his presence and to break the spell that she
casts:

O
listen! for the vale profound

Is overflowing with the
sound.

Thus, although it is
not specified who the words are addressed to, we can infer that they are spoken to
either companions who are with Wordsworth or any other rambler who happened to be
walking in the same spot.