This is off-topic, but I've been running the server portion of OpenVPN at the remote offices, listening on the failover gateway, and running the clients at the central site.
I add this to the client config at the central site:
remote rmt.fai.ovr.con pporrtt;
keepalive 1 4;
Seems to work pretty well.
Total time to failover = failover timeout configured on gateway group + failover timeout configured by the keepalive statement on the client
I believe the above OpenVPN timeout is set to 4 seconds