Hello and welcome to the Communities.
You could do one of two things. You didn't mention if you already have your Google calendars within your Mac's Calendar app. It's perfectly possible to have them co-exist and you can operate both from the Calendar application.
However, if you want to discontinue your Google calendars completely you can export them from Google as a calendar file, then import that file into the Mac Calendar app.
Google explains how to export in the support page Export your Google calendar.
Apple explains how to import in Import or export calendars on Mac. Scroll down to the section called: Import events from a calendar file.
Note the advice in the latter to create a new calendar in the Calendar app to receive your imported Google data. Once your Google data has been imported to that calendar you can then, if you wish, work within that calendar to move events to any of your existing calendars in your Mac Calendar application.
Do this by turning off the visibility of all other calendars so that only your new (and former Google) calendar is visible. This will make it easy to identify these events quickly. To do this choose the calendar icon at the top of the Calendar app. This is the left-most of the two icons next to the three 'traffic light' buttons. Then remove the tick in each of the calendars presented except for your new calendar with your imported data.
To move an event to a different calendar is just a matter of opening the event and selecting a different calendar. This is the toggle with a colour at the top right of the event's panel you will see when you double click on the event.