In your Kentico stylesheet for a site, make the Google font reference by adding a URL reference at the beginning of the stylesheet code:
For example, this references the font and additional lines reference external stylesheets used by the site:
@import url(http://fonts.googleapis.com/css?family=Nobile&subset=latin);
@import url('../App_Themes/MySite/Styles/reset.css');
@import url('../App_Themes/MySite/Styles/960.css');