There are two ways for how to set this up, depending of your current iClosed setup.
#1 Short-term solution
This solution is used for a specific day or block of time on a particular day.
Locate iClosed event/s in your Google Calendar and click on it
Click on the "Pencil" button to edit
make event "Free" instead of "Busy"
Hit "Save" to confirm
iClosed will then automatically count you as "unavailable" for this particular time slot, depending of the choice.
#1 Long-term solution
Utilize sub-calendars in your Google Calendar.
In case you don't have sub-calendar yet, create it real quick by following next steps:
Navigate to your Google Calendar
Click in the "+" button to add a calendar under "Other Calendars"
Set the name and time zone of the sub calendar
Continue adding private events in sub-calendar
To exclude private events, stored in your sub-calendar from conflict check
Navigate to β Calendar under AI Scheduler on the left side menu
Check for conflicts section β and hit "Edit" button
Unmark sub-calendars to be checked against Availability conflicts