Hi David,
if I got it right, you want to handle some outgoing traffic through a virtual server.
As a resource a pool with a couple of gateways is applied.
Probably your virtual server is a wildcard 0.0.0.0/0 in PerformanceL4 mode.
To make sure outgoing requests from the same internal client will always go through the same gateway (may be required, if the gateway applies source NAT) a source address affinity profile needs to be selected in the virtual server´s resource settings.
This approach may not fit, if the internal requests are forwarded by a proxy. In this case the source IP will always be the same ... external proxy IP. As a result all requests would be send out through the same path.
Alternatively you can select a customized destination address affinity to make sure requests to the same destination will go through the same gateway.
Thanks, Stephan