GTM iRule defining external DNS server

Yes, you would enable recursion (beware, note the CERT VU800113 published a couple of weeks ago) and set up forwarders in your bind configuration (GTM->ZoneRunner->Named Configuration)

  
  options {  
      forward only;  
      forwarders {  
          ns1.internal.company.com;  
          ns2.internal.company.com;  
      };  
      recursion yes;  
  }; 
  

You don't need an iRule for this to work, but you could write one to only forward valid requests, which would require you to list in the rule each request not handled by the GTM that your internal servers would have an answer for. The need for this would depend on your business (and security) rules.