SQL Service Using Group Managed Service Account does not start after reboot
The information in Using a gMSA with SQL Server by Wayne Sheffield worked for me with the service issue.


  The pitfalls of using a gMSA with SQL Server
  
  As with almost all things, there is inevitably something that doesn’t work correctly. One thing that I found is that when the server is rebooted, the SQL Server services are not restarted. And I’m not the only person that has had this problem. After digging through the Windows event logs, we were able to figure out the issues. The service was not restarting because it was trying to do things before those services had been started. We were able to overcome this issue by creating service dependencies. A service dependency prevents the service (SQL Server) from starting up before other specified services are running.


See the linked article for full details.