The live web application is currently running on one site. New site has to be setup following which the traffic will have to be routed from the old site to the new site. GSS will be implemented for the first time on both sites, old and new.
What is a good way to test out the GSS functionality before going into Production i.e. without affecting the old production internet traffic.
TTL is defined by Minimum field in SOA (Start of Authority) type as default TTL (common practice), or individually at each record as TTL (comparatively rare practice).
So the lowering down of ttl value in your case should happen at you ISP's DNS server, which is authoritative for the domain.