It's possible to modify the Google Tag Manager (GTM) snippet to run synchronously by removing the j.async=true;
line and placing it at the top of your page, immediately after the opening <head>
element. Like so:
<!-- 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.src=
'//www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXX');</script>
<!-- End Google Tag Manager -->
You can also retain support for users without Javascript by inserting the <iframe>
part of the code in the original location specified by GTM (after your opening <body>
element).
<noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-XXXX"
height="0" width="0" style="display:none;visibility:hidden"></iframe</noscript>
It's then possible to insert the Google Analytics Experiments code as a Custom HTML tag in GTM. Just make sure you check the 'support document.write()' checkbox.
Just don't forget to update GTM-XXXX with your container ID.