Use Ninja Forms File Upload fields as the source of an ACF Gallery field, when using the Post Creation addon.

nf_fu_acf_gallery.php

<?php
add_filter( 'ninja_forms_create_post_meta_value', 'my_ninja_forms_create_post_meta_value', 10, 3 );

function my_ninja_forms_create_post_meta_value( $value, $post_type, $key ) {
	if ( 'gallery' !== $key ) { // change this string to match your ACF field name
		return $value;
	}

	return explode( ',', $value );
}

I'm having the same problem trying to import files into ACF gallery from Ninja forms. Where would I need to place this code in order to make it work, please. Would hugely appreciate your help on this.

Save this file to your /wp-content/mu-plugins/ directory (you might need to create the mu-plugins directory).