How to Place Iron port between GTM and LTM, so that request comes to GTM ,will go to iron port then from iron port it goes to LTM, ? is it possble?

GTM and LTM operation and traffic are totally separate, users traffic is not coming to GTM and then forwarded to LTM. it is not like that, GTM will respond to the DNS query and thats it, then another traffic -http,smtp,httpd..etc- will be sent to the LTM by sending the requests to the IP address resolved by the GTM previously and configured on the LTM as a virtual server -static nat usually configured on the router or the firewall.

Why you want to put a mail relay in between? do you want to load balance the SMTP traffic to multiple ironports, or you want the GTM to respond for the DNS queries related to SMTP traffic?