Thanks! I didn't realize that feature was there at all because of lack of documentation for GPE Calendar.

Looks like it supports a Publish/Subscribe model for calendars shared between Google Calendar and GPE Calendar on the N800. You can choose how often to refresh (although I suspect that, like event notifications, it doesn't work unless GPE Calendar is actually running). Or you can click the unlabeled Refresh icon in the Calendars window to refresh manually.

It's a bit of a pain to get that lengthy Google Calendar ICS url into the GPE Calendar url field. After a lengthy process of opening Google Calendar in the browser, getting to Manage Calendars, and displaying the ICS url, I managed to copy the url to Notes and then from there to the GPE Calendar url field.