1. You can select the source ip address or the gateway to be used for the particular WWW Proxy Service that you create - In the example there are three different gateways and I am show editing one Gateway to use a different source ip address.
2. When choosing which ip address to bind the proxy to, you can set it based off the individual ip addresses, or adapter markings ( generally being Internal / External - Internal adapters usually point towards the private network where External adapters usually point towards the internet).
3. Regarding Genie's reference to Web Proxy Auto Detection (WPAD) files, there is more information in the following knowledge base article:
http://support.qbik.com/index.php?_a=kn ... ils&_i=150