WordPress gtag.js with User ID tracking

I think you need to do a couple of things:

Move your calls to get the user inside your function – PHP variables aren’t shared across scopes without using global, which is generally a bad idea because it pollutes the global scope
Consolidate your JS
Use escaping methods before outputting data

With those in mind, I think this might work for you:

<?php
/**
 * Plugin Name: Analytics Suite
 * Plugin URI: -redacted-
 * Description: Implements Google Analytics using the new gtag.js and combining user id.
 * Author: -redacted-
 * Version: 1.0
 */
function ns_google_analytics() {
    $user = wp_get_current_user();
?>
<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXXXXXX"></script>
    <script>
    window.dataLayer = window.dataLayer || [];
    function gtag(){dataLayer.push(arguments);}
    gtag('js', new Date());
<?php if ( 0 !== $user->ID ) : ?>
    gtag('config', 'UA-XXXXXXX', {
        'user_id': '<?php echo esc_js( $user->user_login ); ?>'
    });
<?php else : ?>
    gtag('config', 'UA-XXXXXXX');
<?php endif; ?>
    </script>
 <?php
 }
add_action( 'wp_head', 'ns_google_analytics', 10 );

In the above code:

We get the user outright using wp_get_current_user. From the WordPress codex (Emphasis mine):

WP_User object where it can be retrieved using member variables. Attribute ID will show 0 if there is no user.
This means you will always get a \WP_User back, and you can check if the ID is zero to determine if a user is logged in or not.
Your conditional if statement really only needs to output specific JS in one spot, so those areas are consolidated
All output should be escaped, hence the use of esc_js

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