Your free/busy information is shared with others by default. Get your coffee, you are already done.
Follow the steps to enable free/busy information sharing of Google Calendar account:
Login to your Google account and display a calendar. To change free/busy sharing settings start by clicking on drop down menu icon next to your main calendar (only main calendar can be shared as it is the only one related to your gmail email address) in calendar list on the left. Select Calendar settings from the menu.
On calendar details page, click on Share this calendar tab in the main menu right bellow calendar title.
Finally make sure both checkboxes of Make this calendar public and Share only my free/busy information are checked. Of course you can optionally check only the first one if you want to make your calendar completely public including event details. Do not forget to save new settings by hitting Save button in the bottom of the page.
If you like to setup free/busy information on Zimbra calendaring system, follow next easy steps.
Login to your Zimbra system and choose Preferences.
Select the Calendar option to configure detailed setup.
Choose the suitable setup to use. You can allow internal and external users, only users from internal domains or limit the free/busy for the domain only. You can also specify exact list of users to see the free/busy information.