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Sure, just use the Mail app to add your gmail account. It will offer to setup iCal for you. I'd imagine you can also do the same from within iCal but I found it easier to do everything at once in Mail.
 
The calendar is now showing in my iCal but obviously its empty

i have 2 calendars

1) on my mac
2) gmail

how do i make sure all entries go onto the gmail account NOT the mac one - inc birthdays of my contacts in my address book ..
 
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