<link rel="stylesheet" type="text/css" href="http://localhost/projet/tpl/fonts/fonts.css" />
What you have here attempts to access a file from your local computer (localhost), it isn't working because how can gmail access a file on your computer? they can't!
You would need something like this:
<link rel="stylesheet" type="text/css" href="http://my-website.com/projet/tpl/fonts/fonts.css" />