vserver ALWAYS replies to ping

I also prefer to use iQuery to monitor from GTM->LTM, but you should take into consideration your architecture. Something like iquery probably wouldn't get messed with in firewall rules, but 80/443 rules do quite often...having a poll on the real service provides an additional feel good that firewall rules aren't getting mangled, or other potential problems. If GTM/LTM sit in the same DMZ layer and would traverse nothing, additional polls wouldn't make much sense.