It would be nice if you wrote about what you are referring to.I assume https://docs.opnsense.org/manual/how-tos/multiwan.htmlThe rule is placed above (before) the default allow rule on each interface that uses the gateway group.To be honest: I never understood this rule, but never cared about as I have such a rule anyway for redirecting DNS.What are you intended to do?For Failover only, you need to use different tiers, where the main gateway ist the lower one.
As said, I never understood this DNS rule... Maybe someone other can explain it...Gateway groups are not really neccessary for failover multi WAN, it works fine using GW priorities. With GW groups and policy based routing you are just a little more flexible for some scenarios.