How to restrict Admin from creating new users from Add new user screen in dashboard to only of one domain?

Question

You can hook into registration_error and check if the user’s email is under your specified domain or not:

add_filter( 'registration_errors', 'user_email_checker', 10, 3 );
function user_email_checker( $errors, $sanitized_user_login, $user_email ) {
    if ( !preg_match('#[@.]gmail.com$#i', $user_email ) {
        $errors->add( 'invalid_email', __( 'ERROR: Only "gmail.com" email address allowed.' ) );
    }
    return $errors;
}

This way you don’t have to deal with the core directly.