@j@svg:
One more question, at what point is the gateway admitted back into the group?
I'm not 100% sure, but I believe the previously-failed gateway is put back in the group/pool after it comes back up.
This entry in the manual may be useful to you.
Lastly, mostly unrelated but somewhat relevant, there seems to be a widely-reported bug with 2.1-RELEASE that some users (including myself) experience, where if a WAN interface loses its connection (say, if the cable comes unplugged) when the connection is restored pfSense gets stuck continually rebooting the NIC and will never re-establish the connection, until you power-cycle the whole box. So, if you're on a flaky ISP, you may want to hold off loading 2.1-RELEASE until they've addressed the issue.