I have tried to do that, and got perfect result.
You need to do two things,
First, make sure the Watch is connected with your phone, make app running on the Background.
Secondly, From iPhone Google Calendar app go to the event > Click on Edit icon > tap notification > tap custom or choose any notifcation according to your preferenes.
Remember, When you will get a notification on your iPhone, you will get on your Watch too.
Hope this helps!!