You wrote:
We support it, and now if you complete/archive tasks (including tasks synced from Google Cal) in Doit.im, there will be a "√" checkmark in front of then on Google Cal. You may have a try now.

If you have more questions about it, please keep in touch.

Thank you for your great support.

--> It do not work for me!
In addition, i would like to checkmark within google cal directly - without opening doit.im

KR