Hello HS,
ther's a simple way to add a proxy arp entry to a gateway without configuring via the GAiA portal/ clish or using local.arp
Add a host object with your external IP to your rulebase and configure automatic NAT (static). As NAT-IP use the same external IP, add the relevant gateway and do a policy install. With this host object the gateway adds an proxy arp entry to the the gateway with the correct MAC.
We use this with loadsharing bond and active/passive bond and VSX. It works too with vMAC.
Wolfgang