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?

I dont' think this is complicated. Although GTM works directly with LTM, from a client traffic/IP perspective they are entirely seperate. A client will send a DNS request to GTM (directly or indirectly), get a suitable DNS response with an IP address of an object on LTM and then establish an entirely different connection to a Virtual Server on LTM, using the IP address supplied by GTM.

Where you place Ironport will depend on what operating modes Layer2, Layer4, whatever it supports but from what I can tell, you should ignore GTM and place it 'in front' of LTM although I have a feeling 'behind' may be better.

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?