Horizontal line | WordPress.org

1. If you want to separate the reviews with a horizontal line, you could use some custom CSS like this:

.glsr .glsr-review + .glsr-review {
  border-top: 1px solid currentColor;
  padding-top: 2em;
}

And this is what it looks like:

2. If you were talking about the rating summary bars (not the reviews), you could do it like this:

.glsr .glsr-bar + .glsr-bar > * {
    border-top: 1px solid currentColor;
}

Which looks like this: