If you run
C:\> git config --list --system
credential.helper=manager
and get a setting for credential.helper
like above, it will have precedence over gcloud installed credential helper.
You can unset it via (possibly as administrator)
C:\> git config --system --unset credential.helper
Now
C:\> gcloud source repos clone default my_repo
should work and
C:\> cd my_repo
C:\my_repo> git config --list
should display
credential.helper="gcloud.cmd"