What scope of detail are you looking for? The addition of GSS is simply controlling the A record that is returned to the requesting DNS client. GSS is using backend processes to track the availability of the hosts and only replying with A record that is currently available. Once replied, the interaction with the web site is the same as without GSS.
Cisco GSS 4492R Global Site Selector
"The primary role of Cisco GSS is to implement the business continuance and disaster recovery policies of a business by optimizing and securing the Domain Name System (DNS) infrastructure of the data center. It does this by integrating with the DNS infrastructure and responding to the client DNS requests, thereby directing the client to the site that is best able to serve its needs. Cisco GSS optimizes its responses to client requests based on site availability, geographical and network proximity, enterprise policy, current load, and available capacity. In addition, Cisco GSS comes with an optional DDoS protection function that substantially mitigates the risk to application availability when faced with a DDoS attack."
"Cisco GSS uses the proprietary Cisco Keepalive Application Protocol (KAL-AP) to communicate with Cisco ACE10, ACE20, and ACE30 modules or the Cisco ACE4710 appliance to offer enhanced and detailed monitoring. Cisco GSS uses this information in conjunction with set business policy to select the data center, application server, or cloud that is best able to serve the user request within user-defined service levels."
"Data center and cloud administrators can use the robust functions offered by Cisco GSS to enable a dynamic infrastructure that adjusts to the changing LAN and WAN environments. In addition to automating failover policy to counter a catastrophic event such as a data center outage, Cisco GSS allows policy-based intervention. For example, using the licensed geolocation services function, a data center administrator can get Cisco GSS to direct a client to the data center or cloud that is geographically most desirable."
Globally Load Balancing with the GSS
Once the GSS becomes responsible for GSLB services, the DNS process migrates to the GSS. The DNS configuration is the same process as described in the "Request Resolution" section. The only exception is that the NS-records point to the GSSs located at each data center. The GSS determines which data center site should receive the client traffic.
As the authoritative name server for a domain or subdomain, the GSS considers the following additional factors when responding to a DNS request:
•Availability—Servers that are online and available to respond to the query
•Proximity—Server that responded to a query most quickly
•Load—Type of traffic load handled by each server in the domain
•Source of the Request—Name server (D-proxy) that requests the content
•Preference—First, second, or third choice of the load-balancing algorithm to use when responding to a query