Custom WooCommerce payment gateway not showing up - woocommerce

Question

I created a custom WooCommerce 3.0 payment gateway to process credit cards:
add_action('plugins_loaded', 'wc_custom_gateway_init');
function wc_custom_gateway_init()
{
/**
* custom Payment Gateway
*
* #class WC_Gateway_custom
* #extends WC_Payment_Gateway
* #version 1.0.0
* #package WooCommerce/Classes/Payment
*/
class WC_Gateway_custom extends WC_Payment_Gateway
{
public function __construct()
{
$this->id = 'custom';
$this->icon = null;
$this->has_fields = true;
$this->method_title = 'custom';
$this->method_description = 'custom payment gateway';
$this->init_form_fields();
$this->init_settings();
}
/**
* Initialize Gateway Settings Form Fields
*/
public function init_form_fields()
{
$this->form_fields = [
'enabled' => [
'title' => __('Enable/Disable', 'wc-gateway-custom'),
'type' => 'checkbox',
'label' => __('Enable custom Payment', 'wc-gateway-custom'),
'default' => 'yes'
],
// This controls the title for the payment method the customer sees during checkout.
'title' => [
'title' => __('Title', 'wc-gateway-custom'),
'type' => 'text',
'description' => __('Credit Card', 'wc-gateway-custom'),
'default' => __('Credit Card', 'wc-gateway-custom'),
'desc_tip' => true,
],
// Payment method description that the customer will see on your checkout.
'description' => [
'title' => __('Description', 'wc-gateway-custom'),
'type' => 'textarea',
'description' => __('Credit Card', 'wc-gateway-custom'),
'default' => __('Credit Card', 'wc-gateway-custom'),
'desc_tip' => true,
],
// Instructions that will be added to the thank you page and emails.
'instructions' => [
'title' => __('Instructions', 'wc-gateway-custom'),
'type' => 'textarea',
'description' => __('', 'wc-gateway-custom'),
'default' => '',
'desc_tip' => true,
],
];
}
/**
* #param int $order_id
*
* #return array|null
*/
public function process_payment($order_id)
{
$order = wc_get_order($order_id);
// ... API call and various validation logic...
// Remove cart
WC()->cart->empty_cart();
return [
'result' => 'success',
'redirect' => $this->get_return_url($order)
];
}
}
/**
* Add the gateway to the WC list.
*
* #param array $gateways
*
* #return array
*/
function wc_custom_add_to_gateways($gateways)
{
$gateways[] = 'WC_Gateway_custom';
return $gateways;
}
add_filter('woocommerce_payment_gateways', 'wc_custom_add_to_gateways');
}
However, the custom gateway never shows up in the list of gateways on the WC settings page.
What step am I missing here?