Open Source Sync with GMail

So this GCalDaemon looks pretty good.  I haven’t tried it yet.  But I will and maybe post a followup

Two-way update and sync between your desktop
and the web is the ultimate golden ticket; it gives you offline access,
local backup and the rich desktop experience while dealing with data
stored in the cloud. With GCalDaemon running, you can view and update
your Google Calendar from any desktop calendar program - even if you’re offline. Additionally, you can access your Gmail contacts list in a desktop email program or address book like Thunderbird or Outlook Express.

Geek to Live: Sync Google Calendar and Gmail contacts to your desktop - Lifehacker