Google fonts are webfonts, they'll load in your browser but not ALL email clients.
If you open your email using the marketo system view as webpage link (note some email clients are adding their own view in browser links and these may not work), the fonts will work - and the marketo previewer is essentially doing just that. But actual email clients are more restrictive in the HTML/CSS it supports, @font-face being one of them, many email clients will essentially re-write the raw HTML so what you're seeing in the email client isn't what you wrote in your email editor
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