You can bring a Mac calendar to your Google calendar by exporting it first to iCal file format.

1. Go to your Mac Calendar.

2. Activate the calendar you want to export from the left side navigation. If the calendar list isn't displayed, click Calendars from the up left corner.

3. Click File in the top navigation panel and choose Export > Export...

4. Choose a location and a name for the exported file.

5. Click Export and the file is downloaded.

6. The file containing the events from your Mac calendar is now ready to be imported into your Google Calendar.