To sync your Google calendar, go to Account Settings and choose "Calendars". Click the 'Connect Google Calendar' button and follow the prompts. We also have this video that helps walk you through the process of connecting your calendar.
Please note: You can only sync ONE Google Calendar at a time in 17hats. Attempting to connect an additional google calendar will unsync any successful connections you already have. You can create and sync multiple sub-calendars through the 1 connection however. To learn about our workaround for connecting more than 1 Google Calendar using Sub-calendars, click here.
We do not currently support iCal/calendars app, but the video will provide instruction on connecting your iCal to a Google Calendar (OS X only).