This article describes how to use Google calendars together with Lightning. You can use Google calendars in two ways. You can either have read-only access to public calendars or you can use a bi-directional calendar, with both read and write-access.
If you only need read-access to a Google calendar, you can use the link of the public ICS file which is provided by Google, as indicated on this Google Help article: View from other applications
If you need both read and write access to your calendar, you need to install the Provider for Google Calendar from addons.mozilla.org.
To import the calendar into Lightning:
There's still some limitations with using the google-calendar, please see the GData provider page on wiki.mozilla.org.