This Applet watches a specific iOS Reminders list for when you complete an item. It's set to your "Reminders" list by default.
If
Reminder completed in list
This Trigger fires every time a new reminder is completed in a list you specify.
Then
Quick add event
This Action will add an event to your Google Calendar. Simply include a detailed description of when and what.
Explore more great ways to automate iOS Reminders and Google Calendar