JSFiddle - React, Tailwind, and code Playground

HTML

<p>You need to view the source of the results pane in order to see the modified rel attribute.</p>

<ul class="gallery">
    <li><img rel="gallery-group" />g1</li>
    <li><img rel="gallery-group" />g1</li>
    <li><img rel="gallery-group" />g1</li>
</ul>

<ul class="gallery">
    <li><img rel="gallery-group" />g2</li>
    <li><img rel="gallery-group" />g2</li>
    <li><img rel="gallery-group" />g2</li>
</ul>

JavaScript

jQuery('.gallery').each( function(g) {
    $('img', this).attr('rel', function(i, val) { return val + '-' + g });
});