We are planning to replace one External DNS server with two GSS appliance. Can the GSS appliance be hosted at ISP cloud?
If it can be, what will be activities that will be done on the ISP side?
You have two options
1. Leave the existing DNS server authoritative for example.com and delegate the subdomains that are needed to be globally loadbalanced to the GSS. For example if you want www.example.com & www2.example.com to be globally loadbalanced then simply delegate www.example.com & www2.example.com to GSS.This way apart from the GSS delegated subdomains all the other queries will be
served by the Primary DNS server.
2.Make GSS the authoritative for Domain and use NSforwarding for unsupported/not configured answers.