This is more or less an extension on this help page which explains how to show your tasks in Google Calendar.

But I'm not interested in showing all my tasks, it makes my calendar look cluttered. I would only like to see important tasks (deadlines for instance). For that, I use a special tag gcal to mark those tasks. Then I create a smart list with tag:gcal, take the iCalendar URL of the list at the right (the Events variant) and add it to Google Calendar.

Note that your changes won't be visible right away, Google updates your calendar only once or twice a day. But of course this trick also works for any other calendar application / service capable of retrieving iCalendar resources online.