Persist iRule based on URI

Since you are trying to insert a persistence record, rather than use an existing one, I think you need to use "persist add", and place the command AFTER the load balancing decision, so it can add the results of the load balancing decision to the persistence table:

if { [URI::decode [string tolower [HTTP::uri]]] contains "gb=3" } {
  pool webpool member 1.1.1.3 80
  persist add source_addr 1800
  log "Persist Setup to Member 3"
}

(also, FYI: "use pool X" is legacy v4 syntax which at some point will be deprecated. For forward compatibility, use instead "pool X")

HTH

/d