amp-nextgen-gallery-compat.php

<?php
/*
 * Plugin Name: AMP NextGEN Gallery Compatibility
 * Plugin URI: https://gist.github.com/westonruter/99b1ca0d593f44888401a396d991e3a6
 * Description: The NextGEN Gallery plugin has a resource manager that makes AMP responses invalid. It needs to be disabled in AMP.
 * Author Name: Weston Ruter
 * Gist Plugin URI: https://gist.github.com/westonruter/99b1ca0d593f44888401a396d991e3a6
 */

/*
 * The following is not bulletproof. In particular it would not work in Native AMP mode, or if you have a page called "amp".
 * This runs at init priority -2 because -1 is init callback that looks for NGG_DISABLE_RESOURCE_MANAGER constant.
 */
add_action(
	'init',
	function() {
		if ( ! function_exists( 'amp_get_slug' ) ) {
			return;
		}
		$is_probably_amp = (
			(
				isset( $_SERVER['REQUEST_URI'] )
				&&
				preg_match(
					'#/' . preg_quote( amp_get_slug(), '#' ) . '/?$#',
					wp_parse_url( $_SERVER['REQUEST_URI'], PHP_URL_PATH )
				)
			)
			||
			isset( $_GET[ amp_get_slug() ] )
		);
		if ( $is_probably_amp ) {
			define( 'NGG_DISABLE_RESOURCE_MANAGER', true );
		}
	},
	-2
);

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c