system.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="payment">
            <group id="sample_gateway" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>Sample Gateway</label>
                <field id="active" translate="label" type="select" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="0">
                    <label>Enabled</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
                <field id="title" translate="label" type="text" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Title</label>
                </field>
                <field id="merchant_gateway_key" translate="label" type="obscure" sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="0">
                    <label>Merchant Gateway Key</label>
                    <backend_model>Magento\Config\Model\Config\Backend\Encrypted</backend_model>
                </field>
                <field id="debug" translate="label" type="select" sortOrder="85" showInDefault="1" showInWebsite="1" showInStore="0">
                    <label>Debug</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
                <field id="payment_action" translate="label" type="select" sortOrder="110" showInDefault="1" showInWebsite="1" showInStore="0">
                    <label>Payment Action</label>
                    <source_model>Magento\SamplePaymentGateway\Model\Adminhtml\Source\PaymentAction</source_model>
                </field>
                <field id="allowspecific" translate="label" type="allowspecific" sortOrder="130" showInDefault="9"
                       showInWebsite="1" showInStore="0">
                    <label>Payment From Applicable Countries</label>
                    <source_model>Magento\Payment\Model\Config\Source\Allspecificcountries</source_model>
                </field>
                <field id="specificcountry" translate="label" type="multiselect" sortOrder="140" showInDefault="1"
                       showInWebsite="1" showInStore="0">
                    <label>Payment From Specific Countries</label>
                    <source_model>Magento\Directory\Model\Config\Source\Country</source_model>
                </field>
                <field id="sort_order" translate="label" type="text" sortOrder="160" showInDefault="1" showInWebsite="1"
                       showInStore="0">
                    <label>Sort Order</label>
                    <frontend_class>validate-number</frontend_class>
                </field>
            </group>
        </section>
    </system>
</config>