I think you are mixing up some network terms/concepts, but I’ll offer a solution to your problem and some networking concepts along the way.
First, the reason we recommend placing the gateway on the management subnet is that when a node promotes/demotes the route to the other peer must not change. If the route does change then the PeerLink will drop and both of your nodes may attempt to promote.
You can safely place the gateway on the VoIP subnet if:
- the addresses of your management NICs of your two nodes are on the same subnet since they will never make use of the gateway; or
- you create a permanent route which tells each node how to find the other across the management network.
So long as you setup your nodes such that management traffic will always go in/out the management NIC then you can configure your gateway/default route however you like.
