Hi Rainer,
I have to agree with Rasmus, the question is, do you really want to sync to google calendar or do you want to sync your smartphone calendar (most likely an Android-based phone) with org-mode?
In case of the last, you might omit using google calendar completely and use another service with caldav support. If you have access to an own server (a PC which can be reached from the internet), you might be able to even install your own solution.
There is an android app [1] which allows syncing of caldav-servers with the Android calendar in the same way like the google calendar (in the background, without any user-interaction after initial set-up). Unfortunately, it is not free, but I would say it is worth the money.
E.g. I use SOGo + org-caldav + CalDAV-Sync to sync between my Android-based phone, a web-based calendar and org-mode. Works great.
BTW. There is another Android app (CardDAV), which does the same for your address book.
Hope that helps a bit
Torsten
[1] CalDAV-Sync beta
CC: Completely OT but it seems you use notmuch too, did you manage to get address-completion working in notmuch?