Hey JohnCrooks,
It appears that you're having a problem adding your Gmail account to the Calendar app, and we're glad to provide assistance.
Use your internet accounts on Mac - Apple Support -- Instead of adding your Gmail account directly to the Calendar app, we recommend adding the account to Apple Menu > System Preferences > Internet Accounts. Once it's been added, you can select which services you'd like to use it for on your Mac.
Set up iCloud features on Mac - Apple Support -- If your Gmail account also serves as your Apple ID, you can go to Apple Menu > System Preferences > iCloud, and turn on the Calendar there.
Feel free to get back to us with any questions or problems you may have.
All the best.