Loadbalance gateway with persistent profil

hi, my problem is complicated and my English is poor ... but I will attempt an explanation :) i use TMOS 11.4

I don't use my Bigip as LB but as Forward Proxy with irule 'Forward Proxy' The default route of my bigip is a pool of gateway. This pool is 3 firewall on stand-alone.

I need that each client use always the same gateway. But, it's not possible to configure a persistent for the gateway-load.

I hope that my explanations will help you to understand my problem. if you need more explanation, do not hesitate.