Is there any document on configuring GSS devices in redundancy ?
AS I said earlier GSS is little different than other appliances with respect to redundancy.
All the GSSs are active and there is no primary/standby concept with GSSs (as all GSS serve records all time).
On your Authoritative DNS servers you add DNS records for your application
gss1.abc.com. IN A a.b.c.d <-- A record for GSS1
gss2.abc.com. IN A a.b.c.d <-- A record for GSS2
Your Authoritative DNS server then send GSS1/GSS2 ip to the client's DNS server in roundrobin basis (Roundrobin is default DNS behaviour when multiple records are in DNS database. This roundrobin can be changed, for example on Microsoft DNS servers "http://technet.microsoft.com/en-us/library/cc787484.aspx")
Since both GSS are active the request could be serve by any GSS.
If one GSS fails then the client DNS server (after not gettting DNS queries answered by the failed GSS) will try the other available GSS.
Syed Iftekhar Ahmed