add-full-size-gallery-param-to-rss.php

<?php
/**
 * Add param to display gallery in full size in RSS
 */
add_filter(
	'shortcode_atts_gallery',
	function ($out) {
		$out['link'] = 'file'; // By defaul show file link in gallery, not attachmnet URL
		if (is_feed() and $_GET['gallery'] === 'full') {
			$out['size'] = 'full';
		}
		return $out;
	}
);