Hallo, I made a test with my Opnsense to see if the gateway changes to tier 2 if I reboot the router of tier 1.
Unfortunaly it does not change although the downtime is about 60 seconds (see attatchment).
I configured the WAN failover according to the documentation (https://docs.opnsense.org/manual/how-tos/multiwan.html)
Gateway groups require policy-based routing to work, they don't change OPNsense's default gateway.
If you want the default gateway to change in case of a gateway failure, you don't need a gateway group. Just enable default gateway switching instead (System: Settings: General).
Cheers
Maurice