As I mentioned in a comment on this thread in August of 2019, zzBots can be used to fully 2-way sync between Asana and Google Calendar.
You can now install this as a bot pack in the Bot Store in zzBots
Install it here: https://www.zzbots.com/store/scFbTfi5Yv39RLmrC
Here is a video of how to install it:
It is our goal to make this 2-Way Sync as convenient as possible. We just recently released this 2-Way Sync as a bot pack that can be installed in the bot store.