When a GossipRouter is blocked and doesn't close its connection,e.g. on a firewall rule or CTRL-Z), then it won't get excluded from the healthy set by RouterStubManager.
Solution: add heartbeating between RouterStub/Manager and GossipRouter:

In RouterStub, we add 2 attributes (in TUNNEL):

heartbeat_interval
timeout

So the clients send heartbeat messages to the GossipRouters they're connected to (excluding ones in the reconnect stage). After not having received a heartbeat (or regular message) for at least timeout ms, the connection to the GR is closed and it is added to the reconnect list. This means that, after this time, the traffic would be routed through the healthy GRs only.