Google Tag Manager with CMS 9. The Site module is extended to allow site admins to edit the GTM code

setup.typoscript

page {
    headerData.50 = TEXT
    headerData.50 {

        if.isTrue.data = site:googleTagManager
        stdWrap.replacement {
            10 {
                search = ###CODE###
                replace.data = site:googleTagManager
            }
        }

        value (
            <!-- Google Tag Manager -->
            <script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
            new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
            j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
            'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
            })(window,document,'script','dataLayer','###CODE###');</script>
            <!-- End Google Tag Manager -->
        )
    }


    3 = TEXT
    3 {
        if.isTrue.data = site:googleTagManager
        stdWrap.replacement {
            10 {
                search = ###CODE###
                replace.data = site:googleTagManager
            }
        }

        value (
            <!-- Google Tag Manager (noscript) -->
            <noscript><iframe src="https://www.googletagmanager.com/ns.html?id=###CODE###"
            height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
            <!-- End Google Tag Manager (noscript) -->
        )
    }
}

sites.php

<?php

# placed in EXT:site/Configuration/SiteConfiguration/Overrides/sites.php

defined('TYPO3_MODE') || die('Access denied.');

call_user_func(
    function ($table) {
       
        $GLOBALS['SiteConfiguration'][$table]['columns']['googleTagManager'] = [
            'label' => 'Google Tag Manager',
            'config' => [
                'type' => 'input',
                'placeholder' => 'GTM-123456',
            ],
        ];

        $GLOBALS['SiteConfiguration'][$table]['types']['0']['showitem'] .= ',--div--;Custom,googleTagManager';
    },
    'site'
);