This little snippet provides a simple way to a) implement Google Tag Manager with anonymized IP and b) provide a Opt-Out / Opt-In mechanism: gtagOptOutIn();

gtag-gdpr-ready.html

<script async src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXXXXX-X"></script>
<script>

	window.gtagTrackingID = 'UA-XXXXXXXX-X';
	window.gtagDisableStr = 'gtag-disable-' + gtagTrackingID;
	window.dataLayer = window.dataLayer || [];
	
	function gtag() {
		dataLayer.push(arguments);
	}

	function gtagIsDisabled() {
		return (document.cookie.indexOf(gtagDisableStr + '=true') > -1);
	}

	function gtagOptOutIn() {
		if (gtagIsDisabled()) {
			document.cookie = gtagDisableStr + "=-10; path=/";
			window['ga-disable-' + gtagTrackingID] = false;
		} else {
			document.cookie = gtagDisableStr + '=true; expires=Thu, 31 Dec 2099 23:59:59 UTC; path=/';
			window['ga-disable-' + gtagTrackingID] = true;
		}
	}

	window['ga-disable-' + gtagTrackingID] = false;
	if (gtagIsDisabled()) {
		window['ga-disable-' + gtagTrackingID] = true;
	}

	gtag('js', new Date());
	gtag('config', gtagTrackingID, {
		'anonymize_ip': true
	});
</script>