Block URL directoy with irules

Providing you have SSL offloading or bridging configured on your virtual server and a http profile applied then something similar to the following should do what you ask.

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

when HTTP_REQUEST { if { [string tolower [HTTP::uri]] ends_with "/gateway" } { drop }

try as below as well

when HTTP_REQUEST
 {  if { [string tolower [HTTP::uri]] starts_with "/gateway/" } {
log local0. "Rejecting "/gateway/* request" 
   reject
     }
     }