I recently started using Grindstone to track my daily workload, and I've been using a number of calendars on Google to not only share classifications of appointments, but to track historical data (Android call logs, etc). I kind of expected that Grindstone would have iCal/ICS export capability, but sadly it does not appear so. You can, however, export to XML. So I thought I'd write up a Perl script to convert the XML data into an ICS file ready for import into Google Calendars. Feel free to download it and use to to convert your Grindstone XML files into iCal data.
Here's the Perl script: grindstone2ics.txt (Save as .pl, not .txt)
If you are on Windows with ActivePerl installed, here's a batch file that you can drag-and-drop your XML file onto. Place it in the same folder as "grindstone2ics.pl": grindstone2ics.bat
Let me know if this helped you!