This attribute indicates the cost metric associated with reaching the
destination indirectly through a gateway node or gateway link that
is part of the selected route If the selected route leads directly to the
destination node then the routeGatewayCostMetric is set to a value
0. If a route could not be selected for whatever reason then the
routeGatewayCostMetric is set to a value 999999 intended to
symbolize unknown cost. For manual routes the value of
routeGatewayCostMetric is also 999999. This is because
RouteSelector does not verify manual routes thus
routeGatewayCostMetric is unknown.