There isn't a good way to sync calendar info from google calendar to E71. If you have Outlook on your PC, you can download the Google calendar sync to add the entry to your PC mail client and then use Nokia PC Suite to sync the calendar over to E71.
http://dl.google.com/googlecalendarsync/GoogleCalendarSync_Installer.exe
http://europe.nokia.com/get-support-and-software/download-software/nokia-pc-suite/download
If you have the ICS file already, you can add them to your Outlook and then sync over to E71 using the PC Suite application.
Nokia also provide a software to sync calendar on the Mac to E71, but there isn't a way that I can see you could sync Google calendar to the Mac.
http://europe.nokia.com/get-support-and-software/product-support/isync
If you use the calendar in Ovi.com, it can sync calendar entry to and from your E71 directly over network connection using WLAN or 3G. Account is free, just like Google, you can find Ovi.com from the following link:
http://www.ovi.com
Hope this helps.