How to get an instance of a plugin
It turns out, in my case, I'm asking the wrong question.  All that I need is the parent config entity (@bojanz pointed this out in slack), so I was over-complicating the issue:

$gateway = \Drupal::entityTypeManager()
  -&gt;getStorage('commerce_payment_gateway')-&gt;loadByProperties([
    'plugin' =&gt; 'paypal_express_checkout',
]);


This will return an array of gateways, get a specific one:

// Get the first one
$plugin = reset($gateway);
$plugin = $plugin-&gt;getPlugin();


Now you have the $plugin to be used elsewhere.