Plugin demonstrating how to inject GTM for the AMP plugin, in response to support forum topic: https://wordpress.org/support/topic/amp-with-google-tag-manager/

amp-google-tag-manager.php

<?php
/**
 * Plugin Name: AMP Google Tag Manager
 *
 * @package   AMP_Google_Tag_Manager
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Google Tag Manager
 * Description: Add Google Tag Manager (GTM) to an AMP page in WordPress. Including Web Stories pages from the plugin: https://wordpress.org/plugins/web-stories/
 * Plugin URI: https://gist.github.com/JonatasAmaral/c871406c6fab056335d788d8c0923eb3
 * Original Plugin URI: https://gist.github.com/westonruter/2ea25735be279b88c6f0946629d0240c
 * Version: 0.2.1
 * Author: Weston Ruter, Google
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/JonatasAmaral/c871406c6fab056335d788d8c0923eb3
 * Original Gist Plugin URI: https://gist.github.com/JonatasAmaral/c871406c6fab056335d788d8c0923eb3
 */

namespace AMP_Google_Tag_Manager;

/** 👇👇👇 This must be populated here with your appropriate value. Or defined elsewhere (preferably in `wp-config.php`) */
define('AMP_GTM_CONTAINER_ID', '______________________');


/**
 * Print amp-analytics.
 */
function print_component() {
	printf(
		'<amp-analytics config="https://www.googletagmanager.com/amp.json?id=%s" data-credentials="include"></amp-analytics>',
		esc_attr( AMP_GTM_CONTAINER_ID )
	);
}

/** _`is_amp_endpoint` deprecated in favor of `amp_is_request`[see here](https://amp-wp.org/reference/function/is_amp_endpoint/)_ */
$add_amp_gtm = function () {
	if ( function_exists( 'is_amp_endpoint' ) && is_amp_endpoint() ) {
		print_component();
	}
};
add_action('wp_footer', $add_amp_gtm);

// include Web Stories pages: https://wordpress.org/plugins/web-stories/
add_action('web_stories_print_analytics', $add_amp_gtm);

// Classic mode.
add_filter(
	'amp_post_template_data',
	function( $data ) {
		$data['amp_component_scripts'] = array_merge(
			$data['amp_component_scripts'],
			array(
				'amp-analytics' => true,
			)
		);
		return $data;
	}
);
add_action( 'amp_post_template_footer', __NAMESPACE__ . '\print_component' );