Add GA to funnctions.php

functions.php

add_action ( 'wp_head', 'rp_add_custom_js' );
function rp_add_custom_js() {
	$ga_code = 'G-xxxxxxxx';

	$script = "<!-- Google tag (gtag.js) -->
		<script async src='https://www.googletagmanager.com/gtag/js?id=".$ga_code."'></script>
		<script>
		window.dataLayer = window.dataLayer || [];
		function gtag(){dataLayer.push(arguments);}
		gtag('js', new Date());

		gtag('config', '".$ga_code."');
	</script>";

	echo $script;
}