/URI is not working cpmpared to /URI/

If you do a client side capture going directly to the server with the "/gtm" URI, do you see it issuing a redirect to "/gtm/"? And do you see the complete and correct URL in this redirect?

Otherwise, you could either transparently change the URI:

when HTTP_REQUEST {
    if { [string tolower [HTTP::uri]] equals "/gtm" } {
        HTTP::uri "/gtm/"
    }
}

Or explicitly:

when HTTP_REQUEST {
    if { [string tolower [HTTP::uri]] equals "/gtm" } {
        HTTP::redirect "http://[HTTP::host]/gtm/"
    }
}