gSync is another answer to two-way syncing with Google Calendar. On the heels of SpanningSync's "controversial" pricing announcement, gSync posted their answer. For a one time payment of $20 you will be able to get gSync for life.

gSync addresses concerns about SpanningSync. For example, it is a standalone client and does not rely on a third-party server. It also allows you to sync with multiple Google accounts.

That all being said, gSync is still very much in beta and I did give it a try - I've not yet mastered getting it to sync multiple calendars within just one account. I'm reasonably confident I simply did it wrong, though. I'll probably have another go tonight.