Well, let's see, I'll test the Subscribe to my Google Calendar...

If I make some changes in my Google Calendar, then I open GPE Calendar on the N800...
Hmmm, looks like it synced automatically as soon as I opened it...
It seems to have picked up the new items from Google Calendar, and the changed items have changed...but the items deleted in Google Calendar are still there in GPE Calendar. Looks like deletes don't sync, at least in that direction.

I tried the Publish option, but no luck - it's not at all clear how it should work. If I give it my Google Calendar url, that's a read-only link to get information, not a means of inserting new information into the Google Calendar. (I tried it anyway, both with private and public addresses, ICAL and XML formats, and it just says 404 url not found when I try to sync). I could try adding a link to a public calendar in Google Calendars, but what url would I give it for my N800? The IP address?? In that case why would GPE Calendar need a url for a Publish calendar, and what url would that be? Needs documentation!!

Not sure what you're asking about where the ICS url goes - if you go to Tools - Calendars in GPE Calendar, click the document icon to create a new calendar, select Subcribe or Publish as the type instead of Local in the dropdown box, you'll see a url field. That's where the ICS url goes.