GTM: Creating a Wide IP that returns different answers to internal clients

you could create two pools and define your internal addresses in one and your external addresses in the other, then use a GTM rule based on source address:

 
 when DNS_REQUEST { 
   if { [IP::addr [IP::client_addr]/24 equals "10.10.10.0"] } { 
     pool internal_pool 
   } else { pool external_pool } 
 } 
 

If you have bind configured under GTMd, then a simple forward statement should work as well (assume the wip has a pool defined)

 
 when DNS_REQUEST { 
   if { [IP::addr [IP::client_addr]/24 equals "10.10.10.0"] } { 
     forward 
   } 
 } 
 

It's been a while since I've written a GTM rule, so you might need to tune this, but it should help along the way.