B Z,
Where did you install GAM? The default is C:\GAM.
Exit Command Prompt/PowerShell
Go to Control Panel/System/Advanced system settings.../Environment Variables.../System variables
Click Path and then click Edit
Verify that C:\GAM (or the installation directory you chose) is on the list; if not, add it
Save and close out of Control panel
Start Command Prompt or PowerShell
Type: gam version
If this stiil gives you an error, reverify the above steps
Do the following commands, follow directions carefully in each step.
gam create project
gam oauth create
Use a non-admin user email address in the previous step.