Insert Availability in Gmail

If you have the chrome extension installed, the "insert availability" option can be found next to the send button on the bottom of the Gmail compose window.

Open a new compose window in Gmail or reply to an existing Gmail thread. On the top of the compose window, you'll see the chrome extension. Click the button that labeled "meetings".
When you click on the calendar icon, an Outreach pop-up will appear, revealing your calendar.
Select the available times in your calendar.
Available times will be listed in date/time order.