Adaptation of this fancybox-based folder gallery: https://greekdeveloper.com/2018/folder-based-gallery-for-hugo/

foldergallery.html

<!-- /layouts/shortcodes/foldergallery.html -->

<style>
    div.gallery {
        display: flex;
        flex-wrap: wrap;
    }

    div.gallery a {
        flex-grow: 1;
        object-fit: cover;
        margin: 2px;
        display: flex;
    }

    div.gallery a img {
        height: 200px;
        object-fit: cover;
        flex-grow: 1;
    }
</style>

<div class="gallery">
    {{ $path := print "static/img/" (.Get "src")  }}
    {{ $url  := print (.Get "src") }}
    {{ range (readDir $path)  }}
        {{ $src := print "/img" "/" $url "/" .Name }}
        {{/*
            troubleshoot the $src variable as needed
            <!-- <p>{{ $src }}</p> -->
        */}}
        <a data-fancybox="gallery" href="{{ $src }}">
            <img src="{{ $src }}">  <br/>
        </a>
    {{ end }}
</div>