Head over to your SaneReminders page on SaneBox.
Towards the bottom of the page, you'll see a link called iCal Feed. It will look something like this.
1. Copy the link.
2. Go to Google calendar and click the down-arrow next to Other calendars.
3. Select Add by URL from the menu
4. Enter the address in the field provided
5. Click the Add Calendar button. The calendar will appear in the Other Calendars section of the calendar list to the left.