RCP Sample Gateway

register-gateway.php

<?php
function pw_rcp_register_2checkout_gateway( $gateways ) {
	
	$gateways['2checkout'] = array(
		'label'        => '2Checkout',
		'admin_label'  => '2Checkout Redirect',
		'class'        => 'RCP_Payment_Gateway_2Checkout'
	);

	return $gateways;
}
add_filter( 'rcp_payment_gateways', 'pw_rcp_register_2checkout_gateway' );

I am new, and using RCP WP plugin. where Do I exactly put this?