Final code for myCRED Tutorial - http://mycred.me/tutorial/the-buycred-payment-gateway-api/

mycred-gateway-api-tutorial

add_filter( 'mycred_setup_gateways', 'register_custom_gateway' );
function register_custom_gateway( $gateways ) {

	// Using a gateway class
	$gateways['unique-gateway-id'] = array(
		'title'    => __( 'Gateway Title' ),
		'callback' => array( 'gateway_class_name' )
	);

	// or Using a gateway function
	$gateways['unique-gateway-id'] = array(
		'title'    => __( 'Gateway Title' ),
		'callback' => 'gateway_function_name'
	);

	return $gateways;

}

// Purchases are made locally
class My_Custom_Gateway extends myCRED_Payment_Gateway {

	function __construct( $gateway_prefs ) {
		parent::__construct( $args, $gateway_prefs );
	}

	function buy() {

		$this->purchase_header( 'Header / Title' );
		echo 'these things needs to be included on this page';
		$this->purchase_footer();

	}

}

// Purchases are done remotely
class My_Custom_Gateway extends myCRED_Payment_Gateway {

	function __construct( $gateway_prefs ) {
		parent::__construct( $args, $gateway_prefs );
	}

	function buy() {
		$this->purchase_header( 'Header / Title' );
		$arguments = array();
		$this->form_with_redirect( $arguments );
		$this->purchase_footer();
	}

}