Thanks a lot !
It worked :) I changed the & with &amp which made it possible to save :)
We are on v16 and I did not find anything related to tag manager by default in this version. (maybe I did not know where to look for)

Website > Settings > Google Analytics

And it will automatically add this code in your page

<script id="tracking_code" async="1" src="https://www.googletagmanager.com/gtag/js?id=G-XXYYZZ"></script>
<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', 'G-XXYYZZ');
</script>
`