Gateway to back end connection pooling related only parameter is http.max.connection.per.host.and default value would be integer max value. Worker pool size do not have direct relationship with client to gateway connections or gateway to back end connections. They are just processing threads available within server.
So at anytime of you need to change pooling details from gateway to backend you can tunne following parameter in "passthru-http.properties" file.
port = 20