hash-based load balancing; aton() and pool member index's

If you use least connections on your gateway pool and build a child profile to the srcaddr persistence profile with a mask of /24, you shouldn't need an iRule at all to achieve your goals. If one gateway that has persistent records goes down, the persistent information is deleted and new connections will flow to your only available gateway.

iRule comparison below:

when CLIENT_ACCEPTED {
   use pool gate_pool
   persist srcaddr mask 255.255.255.0 
}