Your calendar does migrate automatically, but if you are not confident that your calendar migrated properly or have a secondary calendar, this is how you can manually migrate it from Outlook to Gmail.
To begin, open your Outlook calendar and click File.
Click Save Calendar.
Choose a save location, click More Options.
Under the date range click Whole Calendar. Under the details options choose Full Details. Underneath Advanced check Include details of items marked private and Include attachments within calendar items. Click OK and Click Save on the next window.
Note: In some cases your calendar may be too large to import, in that scenario choose Specify Dates within the Date Range option and break up the file into multiple sections of a few months/years at a time. Import each section individually.
Now open up your Gmail Calendar and navigate to Settings.
Choose the Calendars tab and then click Import calendar.
Finally, Click Choose File and select the file you saved from Outlook. Choose the calendar that you want to import to. Click Import.
Your calendar from Outlook has now been imported into Gmail.