Hi Neil,
The feature you are referring to is Network Proximity. The documentation could be a bit confusing because the "client" is relative. To you and me, the client is the person (or application) making the original DNS request. To the GSS, the client is that person's (or application's) local D-Proxy (DNS server).
When an application makes a DNS request to it's local D-Proxy, the D-Proxy will then attempt to resolve the request, but will source the request with it's own IP address, not the actual client's. Therefore, by the time the request reaches the GSS, the GSS has no information on the original "client" or application that originated the DNS request. It only knows the source IP address of the requesting D-Proxy and the domain it is requesting resolution for. So to answer your question, no, the GSS cannot make a load balancing decision based on RTT between actual "client" and data center because it does not have sufficient data to make such a decision.
In most cases, each actual client will be using a D-Proxy that is local to them, so the effect is the same as what you are asking for. But if the D-Proxy is remote to the actual client, then you could get unexpected results.
Hope this helps,
Sean
