gforms_file_upload_acf_gallery.php

<?php
add_action( 'gform_after_create_post_1', 'lc_gp_media_connect_to_acf_field', 10, 3 );
/**
 * Attach the uploaded images to the created post.
 *
 * @param int   $post_id    The Post ID for the post created with the form.
 * @param array $lead       The Lead Object.
 * @param array $form       The Form Object for the form used to create the post.
 */
function lc_gp_media_connect_to_acf_field( $post_id, $lead, $form ) {

	// Is the gp_media_library perk activated?
	if ( ! function_exists( 'gp_media_library' ) )
		return;

	$file_ids = gp_media_library()->get_file_ids( rgar( $lead, 'id' ), 2 );

	update_post_meta( $post_id, 'gallery', $file_ids );

}