Beside those well known registry location such as HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
Not sure this registry exist since Windows 8 or not, but it took me a while to find it out in Windows 10 that there is this registry location to Enable/Disable programs to run during startup.

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\StartupApproved\Run32

If you need to add this to GPO through GPP, and the key value is type REG_Binary.
You may want to import this registry item, and then browse to it (I added them manually, which didn't work). Or lookup on the internet on how to convert the binary data first.

* this is same as using msconfig to enable/disable startup item.