For certain repositories it's also useful to install a git hook to prevent individuals from pushing without properly configuring name/email in .gitconfig. This assumes you have administrative access to the server, of course.
wait, isn't it one of core purpose of VCS like Git to know who is doing commit ?
I mean how can you not know who did commit ?
Yes. If someone doesn't configure their username or email (using git config), git will try to create an email address using the format <local_username>@<machine_name>.local and will not set a name. If there's no user.name set, and the email isn't connected to a Github account (or isn't an email address at all), Github will display "unknown" as the commit author.