Sharing your Office 365 calendar to Gmail consists of two steps.
1) Log in Office 365, open your calendar's settings and publish it. Take a note of HTML link.
2) Log in to Gmail, open calendar's settings and and a new calendar from URL. Paste the HTML link there and name the calendar with a descriptive name.
Don't worry, if you can't see the calendar events right away. It may take 24 hours for them to appear in Gmail calendar.