IRULE FOR GTM TOPOLOGY LOADBALANCING

Maybe this will work for you.

gtm rule IRULE {
when DNS_REQUEST {
  if { [ matchregion ldns Banglore ] } {
    pool Banglore-pool
  } elseif { [ matchregion ldns Hyderabad ] } {
    pool Hyderabad-pool
  } elseif { [ matchregion ldns Singapore ] } {
    pool Singapore-pool
  } else {
    pool US_Asia-pool
  }
}

create gtm region Banglore { region-members add { subnet x.x.x.x/32 } } 
create gtm region Hyderabad { region-members add { subnet x.x.x.x/32 } } 
create gtm region Singapore { region-members add { subnet x.x.x.x/32 } } 

x.x.x.x is the LDNS for each region/country