The Google Webfonts datatype uses the API from Google Webfonts to retrieve the list of fonts.
The saved value can then be used in the template to set the appropriate stylesheet:
@import url(http://fonts.googleapis.com/css?family=Pathway+Gothic+One);
To get the list of fonts a Google API key is required. the APi can be retrieved from
https://code.google.com/apis/console/