See New Features Rolling Out Soon
If your domain provider lets you redirect your naked domain to the subdomain address (e.g. make example.com redirect to www.example.com) then configure that.
Instructions: Use a Synthetic Record Also see Steegle's Instructions
Note that in Google domains forwarding http:/mydomain.com and https://mydomain.com will work.
Examples: http://k8oms.page https://k8oms.page
Add an A record of @ (or your naked domain) that points to 174.129.25.170 (a free service from wwwizer.com) from https://www.steegle.com/google-sites/how-to/assign-custom-url-domain-personal