> Everything is working fine, with load balance between the links, redundancy > etc... the issue is only with the IP to test if the gateway is up or not
You have to have a SEPARATE IP for each monitor address... If you are going out through the same gateway, than traceroute out on the net somewhere and see if you can ping some local routers along the way. Put their IP in as a monitor address. Pick one with as few hops as you can find.