sample-gallery-shortcode.php

<?php 

// JS doc https://github.com/fancyapps/ui

// Handle the shortcode
function custom_gallery_forminator_html_function( $atts = [] ){

	// get the IDs from shorcode
	$gallery = ( isset( $atts['ids'] ) ) ? explode( ',', $atts['ids'] ) : null;

	if( ! $gallery ){ return; }

    ob_start();

	// Loop using fancybox JS 
	foreach( $gallery as $item ): $desc = get_post($item);
	?>
		<a href="<?php echo esc_url( wp_get_attachment_image_url( $item, 'full' ) ) ?>" data-fancybox="gallery" data-caption="<?php echo esc_attr( $desc->post_content ) ?>">
			<img src="<?php echo esc_url( wp_get_attachment_image_url( $item, 'medium' ) ) ?>" alt="<?php echo esc_attr( $desc->post_content ) ?>" />
		</a>

	<?php endforeach;

	return ob_get_clean();

}
add_shortcode('forminator-gallery','custom_gallery_forminator_html_function');

// Allow shortcode on Forminator
add_filter( 'forminator_replace_variables', 'do_shortcode' );


// Call the scripts, must be improved, this calls everywhere we should apply some conditionals
add_action( 'wp_enqueue_scripts', function(){

	wp_enqueue_style( 'fancybox', 'https://cdn.jsdelivr.net/npm/@fancyapps/ui@4.0/dist/fancybox.css', '4.0', 'all' );
	wp_enqueue_script( 'fancybox', 'https://cdn.jsdelivr.net/npm/@fancyapps/ui@4.0/dist/fancybox.umd.js', '4.0', true );

}, 10 );

// Similar here, must be improved to call only where form is located.
add_action( 'wp_footer', function(){

	?>
	<script type="text/javascript">
		Fancybox.bind("[data-fancybox]", {
		// Your options go here
		});
	</script>
	<?php
}, 9999 );