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.
Insert Availability in Gmail
1. Open a new compose window in Gmail or reply to an existing Gmail thread. Next to the send button, you'll see a calendar icon that is used to insert your availability into your email.
2. When you click on the calendar icon, an Outreach pop-up will appear, revealing your calendar.
3. Select the available times in your calendar.
4. Available times will be listed in date/time order.