See Brightspace Events in Google Calendar

On the Course Home of any course page on Brightspace, go over to the right side of your screen.

Move your mouse to the Calendar widget (circled in red) and click the arrow in the widget heading to open the menu.

In the menu, click Subscribe.

This will open up a Calendar Subscriptions menu.

In the Calendar Subscriptions menu that pops up, you can choose which Brightspace calendar items (e.g. a specific course's assignments only) to take with you using the dropdown (circled in purple).

If you want to display all calendar items in Google Calendar, then don't worry about the dropdown.

Select the URL (circled in orange) displayed below, and copy it.

Go to your Google Calendar.

On the left sidebar, on the Other calendars heading, click the plus sign (circled in blue) to open a menu (circled in purple) that allows you to add calendars.

Click From URL.

In the settings page that has opened up, paste your Calendar URL from earlier into the field (circled in green).

Click Add calendar (circled in red).