I have a template and it has a reference to a Google font like this:
<link href='http://fonts.googleapis.com/css?family=Open+Sans:400italic,600italic,400,600,300' rel='stylesheet' type='text/css'>
How can I download it and set it up to use in my pages which are running offline all the time?
It lets you download every web font of Google and suggests css code for the implementation. This tool also allows you to simply download all formats at once without the hassle.
Ever wanted to know where Google hosts their webfonts? This service might be handy if you want to download all .eot, .woff, .woff2, .svg, .ttf files of a font variant directly from google (normally your User-Agent would determine the best format).
Also take a look at their Github page.