Turning on the Free/Busy settings for your calendar allows others to see when you're available for a meeting or event without seeing the details of your calendar appointments. This is a completely secure way of sharing your availability.
Go to your list of calendars and hover the cursor over your Google calendar. You will see 3 dots appear to the right of the name.
Click on the dots then click on Settings and Sharing.
Click the box for Make available for... and change the settings option to See only free/busy.