How do I determine what Registry Keys a GPO modifies?
Nowadays most group policies are based on ADMX templates. Since they are XML files, you can see their content with a text editor and look at the registryKey and registryValueName attributes (the schema is documented here: ADMX Schema)
However, as the files can easily be parsed, we can find websites that did the job and provides that kind of information, for example: https://gpsearch.azurewebsites.net/
Be aware that deploying Group Policies by manually editing registry keys is not supported and may lead to inconsistent behavior.