protected $_paymentData;
protected $_scopeConfig;
protected $logger;
public function __construct(
    \Magento\Framework\Model\Context $context,
    \Magento\Framework\Registry $registry,
    \Magento\Framework\Api\ExtensionAttributesFactory $extensionFactory,
    \Magento\Framework\Api\AttributeValueFactory $customAttributeFactory,
    \Magento\Payment\Helper\Data $paymentData,
    \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
    \Magento\Payment\Model\Method\Logger $logger,
    \Magento\Framework\Module\ModuleListInterface $moduleList,
    \Magento\Framework\Stdlib\DateTime\TimezoneInterface $localeDate,
    \Magento\Directory\Model\CountryFactory $countryFactory,
    \Stripe\Stripe $stripe,
    \Inchoo\Stripe\Model\StripeFactory $stripeFactory,
    array $data = array()
) {
    parent::__construct(
        $context,
        $registry,
        $extensionFactory,
        $customAttributeFactory,
        $paymentData,
        $scopeConfig,
        $logger,
        $moduleList,
        $localeDate,
        null,
        null,
        $data
    );
    $this->_scopeConfig = $scopeConfig;
    $this->logger = $logger;
    $this->initializeData($data);
}
 public function getPaymentKey(){
   $key= $this->_scopeConfig->getValue('payment/webpay/keyid');
    echo $key;
    exit; 
}
エコー結果:idfrk3-45pfnrkhwneirgplbmisniepssnie:hirtw45 True Key-'p92GBhcQl7TklHOsWcxBk4eOmL6wpQWBG9nT2Qcf'